Actually, the 'full-time producer with a well-equipped studio' part is the most perplexing thing here, given that you're just NOW getting a rather basic analog patchable. I detect hyperbole.
Anyway, the point of modular is this: when you've exhausted the normal potentialities of existing synth architectures, this allows you to toss all of that out the window and start with your own definitions about how to generate and manipulate sound. Even with the plethora of buttons and knobs that a Sub37 has, you're still dealing with a fixed sound generation path. But when there's no fixed path, you're free to define all of that yourself. Now, if you know synthesis methods pretty well, this isn't a problem. But if not...well, there's potentially a BIG problem.
Consider: two different model kits. One, you have step-by-step directions, plastic parts that snap together, a tube of glue, and decals. The other consists of a number of appropriately-sized pieces of wood, a picture of the end-result, and you're expected to have your own tools, paint, and glue/fasteners. Modular is like the second example. You have the parts...but you have to make them into something. Whereas prebuilds give you the parts, the basics, and all you have to do is twist knobs and such. Most anyone can build the former kit. The latter one is for when you've mastered the former variety and know exactly how to turn those basics into something amazing.
Granted, it wasn't always this way, since back when all of this started, the only choice was modular, period. And this is part of the problem, because modular has this 'cachet' from that history that makes it 'sexier'. But it doesn't make it necessary, because these days, for a large percentage of applications, it's not. I use both modular and non-modular synths, myself...some work perfectly for certain tasks, others are perfect for others.
If you want to know 'why modular?', go back and study the history of synthesizers, back into the pre-synth 'classical studio' period first. Know why they were developed in the first place. And then why the first prebuilts came into existence. And how the first polysynths were developed out of those. And so on. And then, why people DIDN'T use them for quite a while...which is also very important. Understand your instrument...which, as a 'full-time producer', is technically the studio but unless you understand its origins, and the origins of key subcomponents, you're not either. Anyone can say that; walk it, instead. Just sayin'.