Clerking is not limited to those intending to become career clerks or judges, no. (There are very few career clerk positions and becoming a judge is so far down the road that clerking for that purpose seems kind of pointless to me.) The general argument for clerking is that it provides you with experience and connections that you can't get any other way. You work really closely with a judge, who serves as a mentor to you; you get exposed to a wide range of areas of law; you develop really great research and writing skills; you get a judge's-eye perspective on litigation; if a trial-level court, you get valuable exposure to how trials work; if appellate, you learn how to be a good
appellate lawyer; you have a judge in your corner who will go to bat for you throughout your career; you have a built-in network with the judge's past and future clerks; you get a gold star to put on your resume.
I don't know if that's "better" experience than you'd get at a firm, but it's different from what you'd get at a firm. (Well, you may get exposure to different areas of law and develop your research/writing skills at a firm, but in a different way.)
Employment-wise, big firms tend to like former clerks, and there are some employers - particularly the federal government or elite litigation boutiques - who will not consider candidates who haven't clerked. So while you may have a firm offer coming out of law school, if you want to change firms down the line, a clerkship is arguably a useful credential. (Or go into academia - that basically requires clerking as well.)
Now, these are the ideal benefits. They're very litigation-focused - if you know for absolutely certain you're going to do corporate/transactional and never want to set foot in a courtroom, ever, clerking is much less relevant. I've seen corporate people say both that they were discouraged from clerking and that they were encouraged to clerk regardless, so it may depend on where you're working. (I know people who have clerked despite not intending to do litigation, so some people still do it.)
Clerking also may not work out so well in practice. Your experience as a clerk is very much determined by who you clerk for (and with). If you work for a great judge, you will have an excellent experience. If your judge is dumb, lazy, sadistic, or otherwise unpleasant, you will probably be pretty miserable, though it may or may not nonetheless be a valuable experience. (I had a law school prof who clerked for Kozinski - he said when he went to his firm the 80-hour weeks were a reduction in his workload. But I suspect clerking for Kozinski you'd still learn a lot, even if unhappily.) Your co-clerks make a difference too (you spend most of your time in chambers with the 3-5 other people in chambers - it's a very very small environment).
Generally, you don't "lose time" clerking - you get credit for the time you clerked. But how much can vary by firm (I believe - I'm going on anecdotes because I don't have personal experience with this). Two years of clerking will likely be credited, but beyond that it's less likely, and I think some firms don't like to give even two years. I also can't really comment on the experience you miss out on (again, no personal experience) - I've seen people say that clerking allows you to skip some of the worst mindless doc review parts of being a biglaw associate, but I've also seen biglaw associates working with clerks complain about them being staffed on matters as a 2nd or 3rd year without actually having those skills. So this also probably varies by firm. However, the firms that value clerking don't seem to care that you're missing out on those years.
The other issue to consider is pay - there are people who believe that clerking is not worth forgoing the $160K salary for a year. (Federal clerks make ~$60Kish, depending on where you are and how much experience you have; state clerkships vary but are generally somewhat to a lot less than that.) Firms generally give bonuses for federal clerkships (some may for state supreme courts, depending on the state), but not usually enough to make up the difference in pay (unless you're a SCOTUS clerk, maybe). Plenty of people still clerk, then go to firms, believing that the intangibles of clerking make up for the loss in pay, but not everyone agrees. (I went from a federal clerkship to a federal position that pays about $300/yr less than my clerkship, so I'm not the best person to weigh in on this aspect of things.)
However, clerking counts as public service for PSLF, so if you want to go that route, clerking isn't going to hurt you economically.
(Sorry to go on so long, it's just something I've thought a LOT about!)