ICQ nickname is information good for user to identify people in Contact List. Sometimes two friends of yours have the same nickname, in this case it's best to rename one of them. If you don't, you can mistake them.
Nickname can be used to find your friends on server, but some nicknames are used so frequently that you would spend less time asking them for their UIN/ICQ# than by searching in results. Nicknames aren't good for identification for their multiplicity.