Reverb can be a wonderful thing, it can really bring your sound to life, and I always like to have at least a little reverb going on to fatten things up. I do have some amps that don’t have reverb so adding a digital reverb pedal is a no brainer for me in that case, but is there any reason I would use a reverb pedal on an amp that already has reverb? I can think of two reasons right away.
I think most of us have played on a hollow stage that your footsteps echo on as you walk. If your amp has a spring reverb that can be a problem you can get that banging sound of the springs when the springs move around and hit the reverb tank. A great solution is to just turn off the amps reverb and use a pedal.
Another reason to use a digital reverb is variety and control. Sure, a spring reverb sounds great but sometimes variety is a great thing to have. Most Digital Reverbs will have Room and Grand Hall Reverb options, this option will simulate the sound of a natural reverb caused by the size of the room. You can get a huge sound using a Grand Hall option making it sound like you are in a huge room.
The best thing to do is just experiment and use your ears! A variety of reverb sounds can really spice up your tone.