The INSERT section is meant to allow users to insert Rack FX or Stomp FX, like they would with an FX Send/Return.
This does not vary from amp to amp, but this does allow for some really cool sounds. This is not affecting the direct guitar signal going into the amplifier, but it affects the "FX Loop" copy behind the signal. Normally it's suggested to use Time-based FX with an FX Loop to avoid burying your dry signal in reverb or delay for example, but there are a bunch of cool uses for this.
If you like to do rock solos, add a phazer to the INSERT as opposed to the STOMP. It hits just slightly differently and for me it makes it slightly less apparent. If you're a metal guy, try the Metal Distortion as an INSERT as opposed to a STOMP and you'll thank me later