I understand that most siege towers would have been covered in material that would be difficult to catch fire. Once it was brought up close enough to unload soldiers through the top, what stopped the enemy from just continually sending men through it to the top of the wall? I can only speculate that it would have been suicide to send soldiers that close to the wall to enter the tower.
You have to keep in mind something about sieges and battles everywhere, in every era : There's overall numbers ( the attackers usually have the upper hand in that variable in sieges), and you have the numbers at a defined point (for instance having 10 000 soldiers, of which 1000 are on that specific hill). One of the keys to winning is having the numbers in your advantage, and an otherwise outnumbered army can and have defeated other armies by using their full strength against one part of their ennemies armies, and then turning back to face the rest of the army.
So now, back to the siege: By sending only one tower, or at least too few, up to a wall, what you're basically doing as the attackers is pitting yourself against an ennemy who has the entire wall and towers overlooking it to shoot at the soldiers coming in and out of it. They are probably going to get outnumbered unless they managed to get some space. So if you send your soldiers up the tower in this way, you are only having a small stream of your troops going up the wall to be slaughtered.
Let's say this is the wall and the tower
=====O===========0==========0===========0
HH
Os are the watchtowers, == the wall itself, and HH your tower. Even if the attacking army covers the fields entirely, the tower is a bottlneck that gives the defending army the advantage in numbers in that defined place.