It's for the kids and kids love fireworks. Home runs are just the excuse to have fireworks for every kid, not just the ones that come on Friday nights.
Unlike most other team sports, in which teams usually have an equivalent number of players on the field at any given time, in baseball the hitting team is at a numerical disadvantage, with a maximum of 5 players and 2 base coaches on the field at any time, compared to the fielding team's 9 players. For this reason, leaving the dugout to join a fight is generally considered acceptable in that it results in numerical equivalence on the field, and a fairer fight.
Bookmarks