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That is the question! To a modern reader, the statement "The Thebans won because they deployed 50 ranks against the Spartan 12" doesn't make sense on its own. We need more information in order to understand how the Thebans gained the victory. Why would a deeper formation be more effective than a shallower one? And while it's obvious that a formation must have some depth in order to survive contact with the enemy, what exactly is the advantage of having so many ranks? Why go as high as 50, instead of 15 or 25?

As you noted, the traditional explanation has been that the phalanx functioned as a "human power drill" (menschliche Bohrmaschine, as E. Lammert phrased it back in 1899). All ranks physically shoved the front man forward, and a deeper file meant a more powerful push. The Thebans had availed themselves of deep phalanxes before, at Delion in 424 BC (where they deployed 25 deep) and at the Nemea in 394 BC (where they deployed to an unknown depth, but more than 16 ranks). Both times, it had allowed them to break through the enemy line. This suggested to many scholars that there were simple mathematics at work: more ranks, more forward momentum. Later sources, speaking about the Macedonian pike phalanx, seem to confirm the idea by stating that the rear ranks of such formations aided in pushing. In this scenario, the last 30 ranks of the Theban phalanx at Leuktra did exactly what the first 20 ranks were doing. They all put their weight behind the file in a concerted effort to bowl over the Spartan formation.

But there are a great many problems with this theory. A number of those are summed up in the other thread; scholars have objected to the notion of hoplite combat as a literal shoving match since at least 1942. It's difficult to imagine all warriors working together to exert lethal pressure on their own front-rank fighters. It's difficult to explain why they would all bother to carry spears. It's difficult to substantiate a theory that no source describes in unambiguous detail. Hellenistic authors may have believed in the pushing weight of deep pike formations, but no source suggests that a hoplite phalanx become a more effective forward force due to its depth. (Some scholars argue that the hoplite's concave shield was uniquely suited to this form of combat, but that seems to ignore its use in a wide range of situations that would not allow for mass pushing.)

There are further problems when we compare the theory to actual Greek battle practice. If the superiority of a deep hoplite formation was a matter of straightforward and inevitable mathematics, why didn't all Greeks use this tactic? Why would the Spartans risk deploying only 12 deep if they knew it would put them at a serious disadvantage? They were no strangers to deep formations; at Peiraieus in 403 BC they had made use of one themselves to defeat the democratic insurgents under Thrasyboulos. If a deep formation was innately superior, why hadn't it become the default?

The answer is clear: a deep phalanx was not a straight road to victory. Despite the success of the deep Theban phalanx and other deep formations in various battles, the historical record shows that the deeper phalanx didn't always win through. At Syracuse in 415 BC, the Athenians in an 8-deep phalanx took on a 16-deep Syracusan formation and won. At Mounichia in 403 BC, Thrasyboulos formed up his hoplites 10 deep and repulsed the attack of the Thirty, who were deployed 50 deep. At Sparta in 362 BC, the Spartan prince Archidamos led a desperate charge of a handful of Spartans uphill against the full Theban allied army and drove it back. There are individual reasons to account for all these outcomes, but they would distract us from the point: none of them should have been physically possible if the encounter between hoplite formations was a simple matter of weight. Apparently, just deploying more ranks did not guarantee victory. As E.L. Wheeler noted years ago, you can't simply apply a principle of physics to a collision between untrained men who are terrified and desperate not to die.

What's more, a deployment in many ranks came with an obvious downside: the more people you're putting behind one another, the narrower your line becomes. Even if we assume that the Thebans at Leuktra had brought their full levy of 4000 hoplites, their 50-deep formation could have been no more than 80 men wide. This brought with it a huge risk of being outflanked. Xenophon describes in his Hellenika how the deep Theban formation on one end of the line at the Nemea may have given them a local victory, but doomed the rest of the coalition army; it no longer covered enough ground to avoid being outflanked and rolled up by the Spartans. In his autobiographical account of the march of the Ten Thousand, he discusses the dilemma:

If we advance formed up many ranks deep, the enemy will outflank us, using their outflanking wing as they like; on the other hand, if we are formed up a few ranks deep, we should not be surprised if our phalanx is cut through by a hail of missiles and men falling upon us; and if this happens anywhere, it will be bad for the whole phalanx.

-- Xen. Anabasis 4.8.11

In other words, even if there could be strength in depth, there was also vulnerability. The Athenian author explored this further in his historical fiction, The Education of Kyros. Working off of his own experiences and his research on contemporary history, Xenophon brings to the stage an Egyptian pike formation of 100 ranks - a greater depth than is ever attested in any source. While this pike block is initially successful against Kyros' thinner infantry line, it ends up being surrounded from all sides and forced to surrender. Scholars like J.K. Anderson have rightly seen this as a critique of the tactical idea of just stacking hoplites in ever deeper columns. Whatever it was that the men in the rear did, they might have been more useful if they were deployed elsewhere. The depth of a phalanx was a balancing act between the strength and flexibility. Different situations required different plans.

So, if not for the sake of shoving strength and certain victory, why did the Thebans take the risk to deploy so deep? We can dismiss a couple other theories quickly. As you already point out, it makes no sense to assume that all these men were just replacements for the front rank; no army would sustain such losses and still fight. Indeed, no Greek commander would willingly treat his own men – his own fellow citizens – as mere cannon fodder, and calculate that thousands of them were worth sacrificing for victory. A.D. Fraser and G.L. Cawkwell argued that the rear ranks served not for pushing, but as a tactical reserve; but this doesn’t make sense unless they were capable of manoeuvring independently, which they never do. In other battles, the hoplite reserve is specifically described as being separate from the rest. Where Xenophon says the Theban formation at Leuktra was “not less than 50 shields”, he must mean that this was a single formation, intended to operate as a unit.

So then why?

 

Continued below