i think the last reason is probably the most releveant, they do it cos everybody else does it.
about JB "tapping" JPM, the contact was tyre-to-tyre, we saw JPM's right-rear deform in the impact, maybe explaining why his suspension wasn't damaged, as the tyre absorbed it.
and DC did similar to JPM in the final corner a couple of years ago, i think that was qualli though. don't think he hit the wall, but it sent his car airborne for a second or two.
in Montreal, Jv famously hit the "wall of champions" on the end of the 1st lap in '97, and Murray thought it was Frentzen.
Murray (in a calm voice): "oh look, there's Frentzen, poor man, he's crashed at the final corner, it's an early bath for Frentzen.
Martin: "it's Villeneuve, Murray"
Martin (now with raised excited voice): "It's VILLENEUVE, IT's VILLENEUVE! OH BOY!!"
RIP Jules, never to be forgotten. #KeepFightingMichael