When the light is on (light off=OD on; light on= OD off), you are locking out 4th gear in your transmission. 99% of the time there is no reason to turn on or off the OD during normal driving.
The only time during normal driving you would need to turn it off is if you are driving up steep hills and the trans
keeps having to kick up/down for 4th to 3rd and vice versa as you drive.
When I went to Kings Canyon, my girlfriends Camry was constantly kicking down while going up the canyon road. Ever 20 seconds or so it would drop a gear (much like when you floor it on flat roads) so it would have the power to go up the incline. Too much shifting back and forth is bad for the trans
. Turning off the OD keeps it from doing that.