You understand the gear set up, right? The half step chainrings are to add an extra gear between the cassette gears. For example the shift from the 5th cog to the 6th cog may be a change from 70 gear inches to 78 gear inches. But the theoretical rider wants to be riding in 74 gear inches. A triple/double setup won't let him do that, thus he has two chainrings that are close together to add an extra jump between rear gears.

And then the granny gear is for steep hills.


I realize that's not very simple, but someone else can explain it simpler