in a word: ANGST
Townsend and the rest of the Who did it for show
Keith Moon amped up the explosives in his drum kit.
Hendrix burnt his to outdo the who at the Monterey Pop Festival
Kurt Cobain / Nirvana
I experienced it a few times when seeing Nirvana. I saw an interview once in which Kurt explained it. This isn’t verbatim, but he said that the instrument destruction at the end of Nirvana concerts served two purposes: if they played a great set, it was a distortion-fueled celebration to end it on a high note filled with noise & feedback, but if they played a mediocre set plagued with mistakes and/or sound issues, it was an outlet to take out their frustration and give themselves & their fans something fun to make up for it. However, the destruction sessions became expected by Nirvana fans at the end of their concerts, so toward the end of their career they turned them into a kind of self-parody by spending increasingly longer periods of time destroying their gear in creative and often sardonically humorous ways to satirize and mock the act (such as Dave Grohl throwing his snare drum in the air and Kurt wielding his guitar like a baseball bat and swinging at it, both repeating this for several minutes).