I can tell you that working customer service, ramp, cargo, ramp tower, operations, ect at both huge hubs and outstations has for sure really given me the big picture. Airlines are very complex, a lot goes on operationally that one can't possibly have a great understanding of without actually doing it all. It also has given me a point of reference for what all these departments go thru, how to help them out, what not to do, how to find loopholes and get what you want\need out of situations, and what is overall most efficient beyond whatever job you do individually. Not to mention that if you love airplanes and traveling, those jobs are mostly awesome.
Once you become a captain and take on all the responsibility that comes with it, I think anything you learn in the airline world by doing it 40 hours per week prior to that will help with the non-airmen decision-making that comes with the gig. When to trust people, when not to, when to put up a fight with CS/ramp because they are fully capable of doing what you need and when to pump the brakes and accept the policies aren't realistically going to be followed, ect. Can't really teach those things, they come with experience. And at the 121 level, especially from the left seat, it'd be hard to understate how many nuances one must become familiar with. I mean, if you care about the "flight" and not just doing your job, anyway.
What I'm surprised no one here has mentioned is that there are plenty of airlines who say if you leave in good standing and come back for a pilot job that you can keep your old employee number (which will usually get you 1st pick in class as to domiciles and types) and some airlines also say they'll most likely grant you an interview if you used to work there and meet pilot mins. When I was at SkyWest well before the hiring boom days, all my coworkers who got their ratings and went back to fly there (even if it was a decade later) were hired during periods of time where the advice was "say yes to whoever hires you first and go" and interviews were far from guaranteed. I've heard several guys at JSX complain that it is hard to get on at United straight from there, but both JSX guys I knew who were both former UA SFO employees did go straight from JSX to UA. So if you have an interest in flying at a company one day, I absolutely disagree that working for that company previously does not help you at least a tiny bit in getting an interview, if not hired.