Why are modern cars designed to be sleek and streamlined?

Answer 1

It is mainly to reduce fuel consumption.

When cars travel forward, they have to cut through the air in front of them. This causes air resistance on the car, which slows it down. The more streamlined a car's design, the better the air flows over the top of (or underneath) a vehicle, which reduces the amount of air resistance acting on the vehicle.

This means that cars can be more fuel efficient. They do not need to use as much energy overcoming air resistance.

* Although this is not as critical for ordinary vehicles as it is for race cars and aircraft, increased air resistance results in the use of more fuel to travel the same distance, and even more at higher speeds. So most newer cars are lower and narrower, and shaped to pass through the air efficiently. Reducing fuel consumption ("improving mileage") also means that smaller fuel tanks are required to travel the same distances, which further improves mileage by reducing vehicle weight.