Simply an expression of pride (or frustration), I'd guess. I don't know that I've posted any online (might have), but if I'm impressed with my results I do tend to bring the target home from the range for a day or two.
It's more self-gratification, and less information sharing or seeking, other than "for me, I can shoot this gun good".
Now, there have been times I've been tempted to post for actual info- I have a Savage 1907 in .32 that I can't figure out how to shoot. If I brace it firmly on a sandbag I can hit cloverleafs on the bulleye at 7 yds. Standing and shooting offhand at that same distance I have "groups" of close to 2 ft. This doesn't occur with any other gun I own, regardless of vintage, caliber, or condition. I concentrate and try to replicate the same grip, stance, etc, but it's like it's shooting knuckleballs instead of bullets. It's just weird. Someday I might post a target for the board's amusement and dissection, if I ever can refrain from tearing one to pieces...