Learning to throw a javelin effectively can be fairly tricky. However a skilled soldier could usually throw a 3-4 lb javelin or spear with significantly more force than an arrow. Caesar described the pilum as being able to pierce through two overlapping Gallic shields at once, pinning them together. Heavier javelins like this were usually used as a shock weapon, thrown at close range while charging towards the enemy.

Light infantry and skirmishers however usually used far lighter javelins, often only 3-4 feet long with a shaft only half an inch thick. These were less powerful than the heavy ones, but would usually be thrown with the use of an Amentum, or throwing string, which worked a little like the American atlatl to give the projectiles significantly more range and accuracy.

Edit: Also, the ability of a javelin to be used one-handed with a shield cannot be understated during a skirmisher "duel". In addition javelins were effectively a fire-and-forget weapon, where as with archery it was fairly impractical to let loose a few arrows and then throw away your bow onto the dirt.

The javelin was not a substitute for archers, it was a substitute for the phalanx.

One important aspect of ancient heavy infantry warfare was how to deal with the enemy's shield wall. The Greek phalanx system was one of the more successful systems, which was to bristle your own shield wall with spears, and press it HARD against the opposing shield wall until it collapsed. This worked well in flat/agricultural land if your enemy was eager to engage on the same terms (which they usually were in Greek states using the same doctrine), but had limitations if your enemy insisted on fighting on steep or rough terrain where you could not maintain extensive rank and file.

The Roman Republic found itself matched against hill tribes and northern "barbarians" like the Celts who were too smart to engage the phalanx on its own terms. These tribes had a completely different system for breaking shield walls, which was to hurl spears and long-handled axes at the wall. These weapons were heavy and thrown with terrific strength, and would embed themselves so deeply into the shields that they would get weighed down, exposing the heads of the Romans. Even if soldiers were strong enough to keep their shields raised with spears and axes embedded into it, the charging enemy would step on the handles of the axes and spears, making it impossible to keep them up. Big overhand sword blows would then come over the lowered shields. The Romans suffered some shocking losses to the Celts in the 4th Century BCE, which led to big military reforms, one of which was the use of javelins instead of spears by a large part of the infantry.

In one sense, then, the javelin was a borrowing and adaptation of this enemy tactic. With the right tactics, the thrown 'spear' turned out to just as effective as the held spear when breaking a shield wall, and more so in terrain where the phalanx didn't work. The Romans took this idea and modified it further: the pilum was designed with a long head that would bend on impact, making it harder to cut away or pull out of the shield, and nearly impossible to throw back. Furthermore, being designed specifically for throwing, it out-ranged the thrown spears of the enemy, allowing the Romans to break up incoming charges before the enemy could hurl their own weapons. The Romans coupled this with a redesigned scutum that was better at dealing with the overhead sword blows of their enemies, and a short stabbing sword that was used low against the enemy's high strike. This system was so effective, it ended up dominating Roman military doctrine for quite a few centuries.

Sauce: Swords and Hilt Weapons, Greece and Rome, Peter Connoly.

There's a lot about the Roman javelin. I'd like to shift the question a bit.

What about the Greek javelins, specifically the mercenary skirmishers employed by Athens, Thebes, Pherae and Macedon.

Javelins are heavier, a javelin stuck in a shield will make it very hard to lift and hold in position. Arrows, not so much.

Javelins are heavier. They could break arms when they hit shields.

They were much deadlier against armored troops, especially chain mail, for the same reason.

They tend to not be as salvageable as arrows, so the enemy won't be sending your own ammunition back at you.

They're somewhat useful as a hand to hand weapon. Not to the extend of a sword or spear, but more so than an arrow.