In the early days of guitar amp modeling, most modelers did not let you separate out the preamp and power amp emulation from the speaker cabinet emulation. This meant that if you played with your modeler through a guitar amp, it would sound small and compressed since the tone itself would include an emulated speaker, and then it would go through a real speaker — in effect, the signal was passing through two speakers. Using a powered full-range, flat-response (FRFR) speaker was the only way to amplify the true sound of the guitar emulation.
These days, however, nearly all modelers allow the speaker simulation to be turned off, so if you want to use your modeler with a guitar amp and speaker cabinet, your tone won’t be compromised.
It really comes down to whether you prefer the sound of your real cabinet or you prefer the cabinet emulation in your modeler. If you like the modeled sound, go ahead and use a powered FRFR speaker with the built-in cabinet emulation of your modeler. If you prefer your cabinet, turn off the speaker simulation in your modeler and use your cab.