Not everybody lives in a major hub, and for those who don't it can be really hard to get a company in one to notice you exist, let alone make you an offer.
Job-hunting in Dallas is hard. I've been unemployed twice. The first time was long-term (to the point where it was affecting my sanity) and during the Great Recession. The second time, which was earlier this year, wasn't that long but still harrowing. After I got laid off, it was two months before I got an in-person interview. I got the job, but I'm fairly sure that a good part of the reason I was even considered for an interview was because one of my best friends works here and put in a good word for me with HR.
Also, on at least three occasions I've tried job-hunting while employed. Only one time did I actually find a new job before giving up.
There is absolutely no reason not to. One might argue this is essential -- how can you know "market" if you don't study the market?
(2) If you're applying to similar jobs (in terms of skill) then a single amount of prep can be shared among many
I know many who try to "get in the market" every year or so just for "curiosity"