If you have no experience in any form of motorsport of any kind, then yes, sims are beneficial. If I hadn't ben playing GT and Forza for a couple years before I started racing school I would've been hopeless. I think they helped a lot. They allowed me to understand what my instructor was telling me.
Once you've been racing for a while, it kinda falls off. I can see the usefulness of sims for learning the basic idea of new tracks, and to some extent fitness (some of the bigger professional rigs give your arms a good enough workout), but you won't learn anything on 'em from a purely driving perspective. That said, it's better than doing nothing.
If you're looking for something to prepare you for the track, then iRacing is probably the best bet, followed closely by LFS. I suggest iRacing because it has the most accurate tracks and you will probably find the one that you will be driving. IMO this is the biggest benefit a sim can offer.