[explorigin]

It adds an element of professionalism to a presentation. You can hook up your lappy to the projector. Once things are tested, you black the screen while you wait for people to file in. This way the introducer can have the audience's full attention before your talk.

Also, it can be a powerful tool when you want the audience to pay attention to you and not the slides (if you need to go off-script for example).

[viraptor]

For the first part, configuring a side screen, rather than a mirror screen works as well. Don't put the presentation on the extra screen until you want to.

Not very helpful for going off script though.