Many of my friends have asked this of me, but I have never been able give a satisfying answer as why The Who destroyed their equipment. I did manage to make one thing clear, nevertheless: whatever they destroyed was their ACTUAL instruments. Gene Simmons also destroyed guitars towards the end of KISS concerts, but that was some cheap-stuff he got from backstage.
So why does The Who destroy their actual instruments? I've did some research on this and came up with some good explanations. First, they wanted to be different from all the other musical acts of the 1960s, namely the Beatles and the Rolling Stones. Secondly, by destroying their equipment, their next will never be the same. In other words, the concert they put together prior to will never be duplicated by anyone! It's similar to saying, "what done cannot be undone." It's like smashing the glasses and plates after having making a toast, so that nobody can back-paddle on the promises made during the toast.
To The Who fans, correct me if you think I'm wrong. Commenting on this is highly appreciated.