I learned about JC back when DMC3 was out.. I would say around 2006-7ish. But I managed to beat the game on DMD mode beforehand without knowing about it. Then I realized that Vergil was pretty easy to defeat JC'ing Beowulf.
I walked into DMC4 knowing about it, and I had a lot of fun figuring out how to cancel moves. But by no means was JC ever a necessary thing. If used correctly, it looks cool and you can do crazy cancels which speed up your combat, but that's it. It could probably make the game easier for you if you mastered it but nothing else.
Now, in DmC, JC is not needed to stay in the air... like at all. However, it helps you switch up your moves a lot faster. For example, if you want to stay in the air indefinitely then you might as well spam the living hell out of Osiris' double-up, Rebellion's roulette spin, or angel lift, but that's gonna give you little to no style meter. If you JC those moves then you might have more chances of switching to another weapon, then do something different, and gain way more style that way. The combat also looks way faster without spamming the chains all the time.