It's not a stupid question.
FR/FR stands for "Full Range/Flat Response". What that means is the speaker doesn't color the sound at all. Or, at least it shouldn't. Reality is sometimes slightly different. But, that's the objective.
Why would you want that? Well, you have a modelling preamp that's doing its best to produce the sound of a particular amp. If you run that signal into a regular guitar speaker, it gets changed. In other words, if you're modelling a VOX AC30 but running it into a Marshall 1960A 412 cabinet, it's not going to sound like an AC30. Same thing is true of most amps. However, if you run your model into a FR/FR speaker, you get a faithful reproduction.
That's not to say that it won't sound good. Some folks like the coloration a traditional cabinet brings and don't care if it doesn't sound exactly like the original amp. Who knows? Perhaps you would rather hear a Fender Twin through a Mesa Boogie 212 bottom. It's not going to sound bad at all. Probably sounds great. It just won't sound like a Fender Twin or a Mesa Boogie.
What I'm sure many guys do is feed existing cabs to see if they're happy, and if so, stick with that solution. Others want to reproduce the original amp/cab sound, so they do the FR/FR thing. Depends on what you're trying to accomplish.