What sample rate do I need to analyze usb 2 with a analyzer?
Nyquist basically says you need at least double the bandwidth (aka the highest frequency component of interest) in your signal to get a sampled waveform unique to your signal. Unique is not the same as accurate. Draw a sine wave with just two points per cycle, then 4, then 8, then 16. You can intuitively see why. Your square wave's fundamental is around 480MHz but the edges of a square wave contains frequencies many times that. But you know it is a square wave so you can weed out things like edges that don't seem to be exactly where they should be if your sample rate isn't many times faster than double.

It is the difference between having a discernible or identifiable picture vs a clear, highly detailed picture. Sometimes, discernible is enough though, especially if you already know what it is you are supposed to be looking at.