From there it's generally quite active (it's a nice diagonal eying the h7/h2 square for instance, and might help control the c-file) yet still quite safe from enemy's minor pieces so that it will not get kicked around losing tempi.
Of course you have to just look at the position, but in quite a few positions this is the case imo.
Title says it all.
I've seen people say that a general rule is that queens belong on the 2nd/7th rank (during the opening?). Especially in sicilian games I see black playing Qc7 alot. I understand why white would play Qd2, but why does black play Qc7?