Online check in for flights can save significant time at the airport because usually you can just go to a special line to check your bags and this line almost always significantly shorter than the main line. imo, it is usually the same length as the elite/first class line, maybe two or three people, depending on the time. A couple flights ago, I got to the airport and at least 300 people were in the regular line during a huge weather delay. I just walked up and dropped my bag having already checked in.

Also I see you are a NW flyer and right now NW is offering 1,000 miles for each time you use online checkin.

For cars, I would guess it depends on your car rental company but it saves you from saying no, no, no, i decline to the upsells for insurance, et al, and often you will be dorected to a row of many types of cars and you can just pick the one you like, regardless of class booked.