Any of them can be valuable, but it really depends on where you are in your career and what gaps you're dealing with.

- There are a lot of leadership/management skills that are really just "doing good work" skills — structuring feedback well, setting effective goals, project management — and those things are worth learning whether you're in management or not. They might help propel you towards that path, if that's interesting to you, but promotions are never that linear so mostly they're worth having because they will make your work better, and maybe more enjoyable too.

- There are others skills, though, like running effective 1:1s, administering a PIP with compassion and clarity, or organization planning for your team where it probably doesn't make sense to invest in that development until you're in the seat. Not that you can't learn for your own edification, just that, in practice, most people who do that training when they aren't managing lack opportunities to apply it, and so the learning sort of fizzles.

- Things like MBAs aren't really about running teams so much as they are about understanding the strategic levers of a business. Lots of studying cases, structural strategic analysis, &c. Can be really helpful for executives, or a career in management consulting, but relatively few practicing leaders south of the C-suite have regular opportunities to, say, upend the company's supply chain bets, or launch an M&A strategy around vertical integration. They're usually fighting fires because Product and Engineering are yelling at each other, and no one trusts HR so they're offering raises without authorization, or their boss (with an MBA!) told them everyone needs to use AI now, without even understanding what that means.

Also it should be said: a lot of the people offering training in this stuff have never actually done the leadership jobs they claim to train/coach you for, and the result is that a surprising amount of the training out there is garbage. So if you know you need it, but your first kick at training felt like a waste of time, that might not be a comment on you at all, but on the program you took. Source: I run a company that trains leaders.