Yes. Fish need a period of darkness just like all animals (humans included). Keeping the aquarium light on 24/7 will also speed algae growth creating more work for you.
You don't necessarily need to set a time and absolutely have to switch the light off right then. As long as it's generally consistent the fish will be fine.
Not really you just have to think this through, does the sun go down at precisely the same time for you, that's obviously unless you live on the equator?
Yes. Fish are like humans, they need light and dark.
How would you like living in light every day, all day for the rest of your life? <-- I wouldnt like it very much.