Coming to the start on the Saturday is a key part of the PBP experience, IMO. Apart from the bike check and picking up your brevet card, jersey, etc., it's great to meet other riders from all over the world. Think of it as the United Nations of Lycra. There's usually lots going on in the velodrome itself and the surrounding town is full of cyclists.
If you're not staying too far away, you'll probably find there are other people riding over to the check-in so you can get a little peloton together. In any case, getting out for a short-ish steady ride the day before is a good chance to give the bike a shakedown and loosen your muscles ahead of the event itself.
It's also worth checking out the first few kilometres of the route and familiarising yourself with the assorted bit of street furniture, traffic calming and roadside drains, which are easily missed (and then not missed!) in the excitement of starting the ride in a huge fast-moving bunch.