At first this question seems to be off topic because any possible answer would be opinion based. However, if we analyse the speed and trajectory of all race winners when crossing the finishing line we could back up an answer with actual data. Of course, such data would be very hard to collect, so we have to use our (sometimes imperfect) memory.
I've been watching F1 races since the 70's. Back then it was very common (actually we could say it was the "rule") to cross the finishing line at full speed. The iconic image that comes to any F1 fan's mind is Colin Chapman throwing his hat in the air:
Look at the speed of the car, with a bunch of people on the track! The one at 7:20 is particularly scaring, completely unacceptable by modern safety standards.
The first driver I remember seeing slowing down at the finishing line and driving next to the pit wall was Schumacher. He started doing that during his Ferrari years, because during his Benneton years it was still common the race official waving the checkered flag at the track, which made impossible driving next to the wall1, as you can see here: https://youtu.be/aklM6-sjdl0?t=73. Then, almost all other drivers started imitating Schumacher and now it's quite common.
The reason? The other answer already explained: celebrating with the crew. By the way, until today I always pay attention to the driver crossing the finishing line, just to see if he will cross at full speed, like Leclerc at the last race on Monza (on that occasion because Bottas was on his tail).
1 Actually, we can argue that slowing down and crossing the finishing line next to the pit wall became possible only after the checkered flag started to be waved at an elevated position. Since the checkered flag being waved at an elevated position was part of the mid-90s safety improvements, even if my memory fails and Schumacher was not the first one, we can determine that drivers started crossing the finish line next to the pit wall around that period.