It's mainly for electric elevators that are in-transit and have a kind of 'queue'. Whilst going down, if someone on the first floor calls it 'down', it will continue to the top floor, then go back down. However, say the elevator is in-transit going down, and someone calls it 'down', it'll stop at that floor before reaching it's original destination. There can be other situations as well depending on how well programmed the system is and that is usually dependent on how many floors the building has.
Things like old-school 'lifts' in coal mines and such just have one switch because there's almost always just 'one-way'.
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I worked at a fancy-shmancy hotel for a while and when I first started I was the bell boy and many times spent my days in an elevator and on more than a few occasions was stuck due to either some dumbass malfunction or flat-out power outages (Seattle, fuck you dude). I became good friends with one of the technicians, actually. Good 'ole Leo. Pastor's son. Elevator expert.
...now there's a random thought....