I might caution all new drysuit divers to resist the urge to use your street clothes as your dive undergarment, for two rather important reasons.
1. street clothes rarely give you the kind of thermal protection you really want. It sucks to end a dive freezing because those blue jeans really have zero thermal protection. Most jogging type pants (fleece) aren't much better. They compress underwater and lose thermal protection.
2. wait for it... SUIT FLOOD. It really, really sucks to have to drive home in soaking wet clothes.
As for the flooded drysuit, it's not if, but when. Even a perfectly good suit with no "issues" can flood. I've managed to put a wrinkle in the neck seal being in a hurry, and also managed to accidentally zip a bit of undergarment in the zipper (again, in a hurry). Both times resulted in a significant flood. Even if you manage to avoid this, eventually the neck seal will wear out, or the wrist seals, or you puncture a dryglove, or the zipper finally starts to leak, or you cut something on a rock or wreck or other...
Having a separate dive undergarment from your street clothes means you have something dry to change into for the drive home.
I also recommend "don't forget to bring a towel!" (nice having dry hair, being able to dry hands etc.) Finally, if you do have a full suit flood, having a spare pair of underwear is really nice.