For me, I have two "CDJs", mostly because I'm kind of shy with my Apps, so I have my apps and writing and a bunch of memes (another thing cdjs are good for) at ~alter_writes but this is more of a personal journal, with a fandom/rp twist? Not sure how to explain it. I do a lot of memes and reading of other people's stuff with this journal.
Another reason to have a CDJ is for friending icon journals!
There's also a slight difference between a CDJ and an Example or PSL journal, but I think it's pretty much the same thing, only I don't comment on anyone with my PSL journal unless I'm working out a line. Either of my CDJ is a "friendlier" name that I feel okay with being known by (referring to the phenomenon of calling people by their journal names rather than their given names) rather than my PSL or example journal.