Center of Gravity is not important only for cars and trucks. It applies to boats and aircraft too. Too much weight to the front or rear or to one side can make the vehicle unstable. Where components such as batteries, motors, and servos are placed or how they are are distributed throughout the vehicle as well as the overall shape and placement of wings, etc. can affect the center of gravity.
With toy-grade RCs, center of gravity is of little concern because they really don't go fast enough to worry about it. With both electric and nitro hobby-grade RCs center of gravity is very important. Sometimes getting the center of gravity correct makes the difference between winning or losing in an RC race.