read the strength theory page, much more detail.
I used to do band pushups as a bench assistance exercise as my shoulders liked it more.
This is good news to me as I still can't bench properly but I have just started doing pushups again and feeling much better than bench.
One thing though, specificity. The more you do something, the better you get at it. On less experienced people, they can probably get some good carryover but if you are benching 200kg or something crazy I highly doubt even a VERY heavily banded pushup is going to prepare you for getting under that sort of weight.