Question regarding the overdrive feature on my car?

Good morning.

Yesterday as I was driving home in my Nissan Versa sedan, I noticed an indicator light reading "OD OFF" on my dashboard. I had no idea what that refers to, and therefore opted to keep driving. When I got onto the highway, I noticed that the engine was revving and making significantly more noise than usual - at approximately 65 - 70 mph, it sounded as if I were going at least 20 miles above that. At this point I became worried, but being clueless about mechanics and not wanting to give in to panic by pulling over and having the car towed, I once again elected to proceed. Finally, I pulled into a supermarket parking lot and consulted the owner's manual (which, granted, I probably should have done right away). In any event, I discovered that "OD" stands for "Overdrive", and that it had evidently been turned off. I couldn't tell where the on/off button for that was based on anything I found in the owner's manual, but eventually located it, roughly three inches down the gear shift lever. Everything went back to normal after that. When I got home, I checked the "Overdrive (mechanics)" article on Wikipedia, and discovered the following: "Overdrive (OD) is a term used to describe a mechanism that allows an automobile to cruise at sustained speed with reduced engine RPM, leading to better fuel economy, lower noise and lower wear." I therefore have three questions:

-Why is there even the option to turn the overdrive feature off? Are there ever any circumstances wherein it's advantageous to do this?

-I trust having driven roughly 30 miles with OD turned off (I still can't for the life of me figure out how I accidentally did this, by the way) isn't likely to have any long-term effects on my car?

-Should this ever happen again, is it safe to turn OD back on while driving, or is it necessary to pull over first?

Thank you in advance, car people, for indulging my cluelessness.