The significant advantage is that you get a new tool to organize your army.
A forced OOB, where you get penaltized for not using it? No thanks. The ability to somehow group my army divisions into 'corps' and be able to give orders to those corps when I feel the need? Yes please.
Also, let's be honest, as long as the AI can handle it, HoI4 probably needs some restrictions on how far apart divisions can be. An army with some of its divisions on the eastern front and the rest in North Africa shouldn't function as well as an army with all its divisions in the same area.
It could be highly useful. For example, you could Control-click a division corps HQ, for example, to select its entire corps, which was hugely useful when moving large numbers of units. On-map HQ units were a pain, and reorganizing after offensives could be a pain (although the picture above my post is of either an AI or some player deliberately not caring about keeping his troops organized -- I could just as well post a picture of an overflowing and disorganized filing cabinet and use it as 'evidence' filing cabinets are bad), but all in all, it certainly did have its uses.
Sad thing is, it really needed only one more small patch to be really useful. For example an auto-organize system that just took all the divisions you had selected and put every five adjacent divisions into a corps, every adjacent corps into an army, and so on, creating HQs as neccessary. And an "assign to nearest HQ" button if HoI3 didn't have one already. And an end to the hard cap of number of units (for example, you couldn't have more than five divisions in a corps).