You would do this especially if you want to forward mail for people for whom you have NOT created an account on your server.
If you have one or two users, then .forward files are fine, but otherwise the necessity to setup accounts and to maintain them is pretty cumbersome compared to using the alias file. And don't forget,
every end user account on your server is another security hole.
Once you are talking 20 or 30 users, forget about doing it manually.
It is simply not a "scaleable" solution.