They are a lot of reasons why someone might want one, even if it does not interest you, personally, which if it is the case, all you have to do is not to download them.

One reason I am interested is that in VR, it gives me the ability to sit in a virtual car and discover what it is like inside.
An other reason is that I am not able to model cars but I can play with the physic characteristic and fix the physic issues or someone else, like a few here in this thread, can.
We are so fortunate that we have so much to choose from that it feels overwhelming some time. We can create our own virtual racing scenario only limited by our imagination. No other sim offers such a wild variety.
Without Content Manager my large cars and tracks collection would feel like a jungle, but with it, i have everything organized and available with a few clicks. I cherish that there is so many sources of mods, take advantage while it last, storage is cheap and you never know when it will dry up, but you know it will.
