There are a few benefits. The biggest one is the lower bandwidth needed for the app. When you are out and about on 3G, the app should be a lot more responsive.
That said, I tend to use whatever mode/app is handiest at the moment when on the iPad. (though I am usually on the iMac, since it's got a big screen, full keyboard, and is about 10 steps away from anywhere in the trailer.)
If I'm just checking to see new posts in my subscribed threads, I use the IPF app. Sometimes when I am bored I'll also use it to check the active topics.
If I'm looking for new threads I use Reeder. I'm subscribed to all the RSS Feeds. When I need/want to reply I use the built in web browser. (you need to change your default editor to HTML in the Edit Options on the User CP page for this to work).
If I'm just fooling around, looking over the site or figuring something out, or searching, I'll use Safari.