In my opinion, black leather looks especially ugly if it starts to crack and fade. If your car seats see a lot of action, you will need to make sure the leather stays protected. A good conditioner will keep it soft and able to handle the abuse of daily wear and tear. If you let the leather...
I've always hated cleaning seat belts, especially if they haven't seen attention in a long time. I've experimented with the Ultima brand and been pretty happy with it. It definitely does a better job than your average generic cleaner.