On a lot of cars, let's say it's a 4-speed auto, overdrive is just 4th gear.

For example my first car, a '90 Legend that my dad handed down to me, had D (picks between all 4 gears) and S3 (picks between 1st 3 gears, holds gears longer) spots you could move the lever to, then it had an S4 button on the top which if you pushed gave you the S gear holding, but also let you go into 4th.

The main reason you would want to disengage it, is if you're climbing up a hill and the transmission is hunting for a gear.