I've found many pictures of Roman pila having spikes on the butt end. Considering how the Romans used to fight in fairly dense formations, the butt-spike was probably very hazardous for the men standing behind the thrower. Plus, you don't really need to plant a javelin in the ground or fight with it in melee (and I really can't think of any other use for that spike). So why did they have that spike?
EDIT: Wow, I learned a lot today. Thank you all, historians of Reddit.
It's not certain the addition of a butt-spike on a pilum was uniform, as a lot of depictions do not make note of it. If it was there, I can't see it being hazardous to the soldiers in the rank behind, as the soldiers are drilled to not get hit in the head by the back of the pilum anyway when it is raised to throw.
If the pilum did have a butt-spike, it would be used for planting in the ground, as it would save the soldiers from having to hold it while waiting for a battle to commence - or in any situation when standing idly. Additionally, if legionaries did in fact take two pila into battle, one would be able to be deposited in the ground whilst the other is thrown.
Source: Goldworthy, Complete Roman Army.
I would like to ask which era of pila are you looking at? Please post a picture of what you are referring to.
Although, I am unfamiliar with pila ever being fixed with a spike at the butt end I understand that they have a heavy and a light pila, and that they have been used to counter cavalry. The pila's main purpose was to cause attrition to the enemy combatants before engaging into melee combat. It could even disable shields as it was cumbersome, and a huge leverage point once embedded into a shield. It went through many versions of designs, but the most important was a design to prevent it from being used by the enemy after it had been thrown. It feels as if it would defeat the purpose of having cheap, light, and disposable throwing weapons that your enemies could not use against you if you attached a spike to the butt end. However, Rome lasted a long time and stretched over a very large territory, and with that came many renditions of things one being the pila. I have a guess that you might be confusing the traditional pila with something of a shot spear which would definitely include a spike or weight at the butt end. Something like the Angon, a Germanic rendition of the Roman pila, or the lanceae used by the auxiliaries. I could be wrong on this. So keep me informed!
While I'm by no means an actual historian, I've done a good deal of reading/practical application on the subject.
Butt spikes are more important and prominent on thrusting spears; while spears are expendable and much less a loss if it breaks than a sword, a well taken care of spear can last a long time. The butt spike would aid in that a great deal, as it effectively protects the rear end from the elements, and when you put the spear on the ground, leaning against yourself(very common, as 99% of the time a soldier is not fighting) the butt spike becomes very important in keeping one end from not falling apart. I suspect that Romans would have reused pila a great deal.
Of also great important is the balance they lend to the spear. Try holding an 8' section of pipe, and try manipulating it in various ways that a spear/javelin may be manipulated. It's not easy. While it's not very big, the butt spike makes it a good deal easier to use spears and javelins.
Just my five cents; feel free to comment or disagree.
In Armor, Sean Morrison says that the spike on the butt end allowed the soldiers to turn the pila around and use them as pikes against cavalry.