Why does PostgreSQL require its own user/group for installation?
The reason is that it is wise to run the database processes under a separate account.  At least one advantage of this is that the OS processes as safer in the case of the database server being hacked.  On the other side the database related files are safer from the 'normal' user accounts too - the PostgreSQL cluster (or at least its data) is relatively easy to destroy with a well-aimed single-file deletion.  

If you check your passwd file you may find lots more similar users for other services, I imagine on similar grounds.