Question for players who play fast in longer time control games- why do you do it?


It mostly boils down to the opening phase. If I can get out of the opening with a solid game, it means I can play safer solid moves, rather than going for complications. It's much quicker to calculate a safe solid line if you know it will keep improving your game, even slightly.
Eventually most opponents will then try to play some wild tactic, but by then, I've banked so much time, I can afford to burn time thinking the opponent's wild play.
On the flip side, against super agressive opponents, that go for a quick early win, that strategy won't work, because I'm forced into defense mode so early and it takes time to think through a fast attack.

Having more time than you started with happens a lot in increment games, even in OTB rated chess. Some people just play fast, all the time.
Back when I used to play in OTB tournaments... the late 1970s, as I recall... there was a player who used to ALWAYS blitz out his moves. No matter who he was playing, no matter what the time-control, he never took more than a minute or two for the entire game.
As you may recall, before the advent of digital clocks they used mechanical chess-clocks, with a little red "flag" that fell as the minute hand passed the 12:00 position. At the start of each game, the tournament director would set all the chess clocks at about one minute before the hour, so that the little red flag was held up by the clock's minute hand. This "free" extra minute prevents anyone from claiming that they hadn't been given the full two hours on their clock.
I was in the tournament hall when this "speed demon" beat a titled player (an FM?) in a 40-moves-in-2-hours classical time control game... forcing his opponent's resignation before his flag had fallen to signal the START of the first minute of play.