You can view AVI files...sometimes.
Trouble is, there is no standard called "AVI." AVI movies can actually be in any one of about...oh, I dunno, 50 different standards or so.
Some of these work on the Mac; some don't. The problem is the "CODEC," or compression technique, the AVI uses. Some AVIs use CODECs that have been ported to the Mac, some don't; some of these standards are patented, some aren't.
So with an AVI, you pays your money and take your chances. In fact, some AVIs won't even play under some versions of Windows.