A completed apprenticeship is like a college degree in the fact that you can go anywhere with it, i.e., it is transportable and people recognize the "value" of the paper it is printed on. You may decide to enter a different trade, but the training and experience are still recognized.
Some companies have in-house training programs that are excellent, but all too often they are not recognized by other companies or in other states. Apprenticeship programs, whether offered by a construction union or a shop, have to be approved by the Labor Department (in many states) and have to meet minimum requirements for hours of classroom training and on-the-job experience. A recognized apprenticeship program is by far the better of the two. The unions do not have an exclusive lock on apprenticeship programs. There are "open shop" companies that have approved apprenticeship programs, but I expect the majority of apprenticeship programs are with union shops. It has something to do with corporate culture and the recognized value of having a skilled work force.
I completed an apprenticeship program with the Ironworkers and both my brothers completed apprenticeships as a machinist and tool and die maker one union and the other open shop). I don't believe any of us would have received the training we did were it not for being part in a state recognized apprenticeship. I have nephews that have been "trained" by their open shop employers, but not part of an apprenticeship program. They are taught how to do a particular operation or how to run a particular machine, but they don't get training on a variety of machines and they don't have to attend evening classes. "Which would you rather have in your wallet?", as the advertisement asks?
Any shop instructor that would steer his students away form a recognized apprenticeship program is an idiot. You can tell him I said so!
Best regards - Al