Cars are heavy. it takes alot of work to get them moving, but not as much to keep them moving. If a car only had one gear, it would start out fine but it's top speed would be very low because engines can only spin so fast. Think of it like riding a bike. When you have it on an easy gear it's easy to get started but you can pedal REALLY fast and not go that fast. If you switch gears once you get going, then your speed increases. If you only have a hard gear, it's fine once your going, but it takes you FOREVER to get stated because it's so hard to pedal from a stop.
There are cars out now that don't have gears exactly. they work on the same principal as a snowmobile where the gear ratio is infinite. So it feels more like your sewing machine, but there are still gear ratios at play.