What is the MacBook Pro's infrared port used for?
Most typically, it is used for the Apple Remote, which can control iTunes, QuickTime and various other built in software. Additionally, 3rd party software (see Sofa Remote and Remote Buddy) also makes use of the device and can tie in much more functionality.

In the past (prior to Lion), the Apple Remote was an excellent companion to Front Row, a media manager that some call the predecessor to the Apple TV interface.