Agree becoming good at your job is number one, but interviewing is an independent skill worth developing. The places I've worked that required interview prep in one form or another, were all around better and all around had higher quality employees. That's not an absolute rule (nothing is) of course. But prepping for interviews gives you more "yes" opportunities to evaluate companies, and once you get competing offers you see something you normally don't. You can get paid substantially more for the same job, without ever negotiating. Merely having other opportunities, your prospective employers will magically offer you more money, a bigger signing bonus, more stock, for the same exact job, and you don't even have to ask for it. You merely tell them all what they are all offering. But of course the real value is being comfortable and confident enough to _take_ multiple interviews, and ask hard questions, and using that to find better companies.

(This of course works for all kinds of things, not just interviewing: Quotes for house work, car purchase / sell offers, etc. Simply get more than one, and poof you get better deals).