One of the many differences to a bus over a group is the way you use effects. Say you've got 5 tracks that make up a drumkit, and you want to put a nice reverb on the kit. You you can put that effect on a bus, then route your 5 drum tracks to that bus, and you don't need a bunch of instances of the same effect taking up resources on your CPU. And since it's one effect being used for several tracks, you can quickly tweak settings for the effect and not have to do it for a bunch of effect instances like you would if you put an individual effect on every track.
There are also tons of ways to use buses artistically too. For example, you can have a dry guitar on a track, pan that to the left, then use the 'Sends' on the channel strip of the guitar to send some of the signal to a bus, put a crazy effect on that bus and pan the bus to the right, and then you've got dry signal left and wet signal right.
Once you understand the basics of routing and signal flow, you'll find this opens up new worlds. Once you grasp these concepts, you'll find them to be very concrete, not abstract at all.
Good Luck!
