Yes there are overlaps ... I use the "big ring" when decending and hammering on flats, and the "granny" when ascending (over 10%, to save my knees). I only use the top 4 cogs on the "granny", the middle 7 on the "middle ring", and the bottom 4-5 on the "big ring". The reason ... cross chaining.

Cross-chaining is not as big a deal these days with the new cogs and chains, but using cheap or older equipment, the wear on the chain and the cogs can make for very sloppy shifting, and pre-maturly wear out your drivetrain.

That is as much as I know ... there are certainly peopleout there that will explain it better than me.