Jailbreaking is handy if you need to get access to apps that ...
a) Apple will refuse to let on to the App Store,
b) were once on the App Store but are now gone (NetShare), or
c) are available for free as unofficial apps but only available as pay solutions on the App Store (all Terminal/SSH apps)
The App Store is cool and all, but Apple is pulling all of the strings. I don't like them deciding that I shouldn't have access to a tethering app, for instance. My carrier allows tethering for the data plan I have.