I am fairly new to guitar and I have been doing a lot of reading up on this site. It says in many places that it is better to have more power in your amp going to your cab than it is to have less power coming from your amp that your cab is rated for. Then why is it that most speaker cabs are rated at something around 200 Watts but most amps only go to 120 Watts?

Amps can usually put out more than they're rated for, but cabs can usually take more than they're rated for. But generally, you want some room for error. So if I'm playing a 100 watt head, I'd ideally like to have a 150 watt cab or higher. But a 100 watt cab will work. Even a 50 watt cab would work, as long as you kept the volume well below half.