Originally Posted by 9duc
I have the mobile app installed, mainly so I get notifications of wall posts, direct messages, etc. It integrates the notifications (if desired) into your messages folder, and also gives a homescreen notification in the banner.
This. I use the FB app so I can make my phone light up/buzz when someone replies to my status updates, messages me, etc. It's also a handy shortcut.
I imagine if I wanted to post a photo I'd taken with my BB, the app would also be quicker.
However, when I want to read through FB I still launch the web app because I prefer the formatting (Menu > Go to Mobile Site
from inside the FB app).