Wikipedia has some more information:
In most early motor vehicles the driving seat was in the middle. Later some carmakers chose to have it nearest the centre of the road to help drivers look out for oncoming traffic, whilst others chose to put the seat on the other side so that the drivers could avoid damaging their vehicles on walls, hedges, roadside gutters and other obstacles. Eventually the former idea prevailed.
The distribution of left-side vs. right-side driving, mapped out:
Occasionally, when crossing a border with a vehicle, there'll be signs to warn and instruct about the change in traffic patterns:
… These signs aren't particularly common around the world, so it's kind of cool to see one online! 👍🏻