The reason to take walls rather than destroy them :)

[The_Emperor]

I find that the main problem with using only Sap points is that you need to blast more than one hole in the wall to be effective... Otherwise the enemy will just mob you on the other side.

Taking the walls is the best way to minimise casualties, and Siege Towers are best for this because (unlike ladders) your men actually manage to survive long enough to get up there.

[Paul Peru]

In my first real siege (beyond picket fences) I managed to get sappers under one of my siege towers
At least it worked as one would expect; the tower collapsed.
Until I read this tread, I hadn't figured out what to do once I was up on the wall, so I just used sappers. (or rams, if I wanted to give my legions a nice, dangerous barbeque)