At first this question seems to be off topic because the answers would be too opinion based. However, if we analyse the speed and trajectory of all race winners when crossing the finishing line we could back up those answers 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:
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 wall, as you can see here: https://youtu.be/aklM6-sjdl0?t=73. Actually, we can argue that only after the checkered flag started to be waved at an elevated position slowing down and driving next to the wall became possible.
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).