It allows you to post anonymously.  Some use an alias when the want to post something that they know will piss off others, so they won't know who said it.  Some use it just because they don't want any posts linked to their real handle, for various reasons.

Swim

and clicking on "my aliases"

When you create the alias, it will be available to use each time you go to post a message from a drop down menu next to your regular user name.

Also, when you use an alias, the envelope next to your name for sending/receiving PMs will not appear.  This is the main way to tell whether someone is using an alias or not.

First off, thanks skb for your answer to my last question.

But now for a separate question, when is the use of an alias frowned upon by one's fellow board members?  For example, I was just reading a thread started by that ScotlandYardDick guy (alias) in the General Discussion forum, and several people there are ripping him a new one for using an alias.  Are they frowning upon his use of an alias simply because the content of his post is so despicable (a confession to writing fake reviews)?  Or do members here generally frown upon people using aliases, except for those occasional instances where they have a very obvious and understandable reason to use one?

Please explain.  Thanks!