The entire concept here is to produce consistent results as easily as possible. Anyone who has spent their fair share of time miking cabs will tell you there is a good bit of luck involved. Every variable matters: the mic, the speaker, the cab, the room, what's going on around the mic, the weather, humidity, etc. This all plays a HUGE roll in getting consistent miked results. Sims remove most of that equation completely.
As for if it sounds like what's coming from your speakers, that's a broad question. It entirely depends on the voicing of the simulator and what speakers and cab configuration you choose to use. If they are not in the same ball park, they won't match. But you will get very consistent results time and again via the simulator, which is very valuable to professionals. Particularly when time is money in a live setting or in the studio.
Speakers and the cab you load them into are the single biggest variable in your tonal chain, hands down. No amount of tube changes, modifications or other tweaks to the amp can accomplish as much as a single speaker swap can.