Shatner does it primarily for humorous effect, I think...and because he can, of course. Perform, I mean...not sing. Even he admits that he can't sing...but he does perform songs in his own unique fashion.

Shatner never tries to do anything, Amos...he DOES things, by golly! But he doesn't sing, that's for sure. He does "exercises in tonal breath control".*

* (an obscure old Dylan quote from the 60s...when some reporters asked him to describe what he does)

View his bizarre rendition of "Rocket Man" on Youtube for undeniable proof of all I have just said.