As with just about every curriculum in any course you'll find nowadays, for every skill or sequence that's taught there's a breakdown of small and smaller building blocks or 'units and elements of competency' in the jargon they use.
The MOS sets out which of these bits are mandatory training for pretty much all the things that are covered by licences, ratings or endorsements.
Whether they are accurate, exhaustive and sensible is a different story, but they're not too bad in the main I think, from what I've seen so far, apart from some dodgy wording here and there.
To address your question directly, the MOS is there to specify in detail what should be taught and tested. Good training is good training no matter how you get there, and what I suppose you're referring to is what we talk about as what happened in the old days, where good instructors used their brains, skill and common sense to teach people how to fly properly and safely.
Now, not just in aviation but in pretty much anything you want to name that has official courses of training, you get this sometimes seemingly ridiculous level of specification of what has to be done. You can't just point out where the toilet is these days, you have to specify how they should unzip their fly, where to point it so they don't splash their boots and how to wash their hands afterwards, it seems.
I think there are pros and cons to this kind of thing - it can get to a silly level of specificity, but it should also help with standardisation and to prop up inexperienced instructors as well as refresh and educate more experienced ones. It makes you analyse what you do against the defined standards, at least.