I've pulled the trigger on the Sony 50mm 1.4

Now I would like to go back to the original question of "telephoto lens". Could someone explain what the prime purpose of say, the previously suggested 70-400mm?

Not that I am in a hurry to spend more on lens. The reason I am asking is because I need to decide whether to go for the Series 2 or 3 of the Gitzo tripod and I'd like to take future lens purchase into account.

I do not do any bird watching (in particular), I do occasionally shoot people from afar (e.g. in street festivals). But it's opportunistic, I doubt that I would carry a 70-400mm and a complementing (heavier) tripod all the time.

I would be interested in shooting fireworks, shooting stars etc. Especially for fireworks, I can prepare in advance, so the question is whether the 70-400mm is significantly better than, say a 70-300mm for such purpose.