These are the most common type of loudspeakers found in automobiles, particularly in the rear doors and along the back deck. Such speakers handle all the signal designated for a particular channel. They usually don't sound too good, and they're cheap, which is why most manufacturers use them. These setups, typically located in the front doors, divide the audio signal into different frequency ranges using an electronic crossover. A crossover is like a cop directing traffic in a busy intersection.
Different instruments play different notes.
Assigning different frequencies to different speakers makes for better sound. There's another reason for doing this, and it has to do with power handling. A separate tweeter can't handle the power needed for deep bass notes. Most car tweeters are about 1 inch across, and have very little mass. They need only a small amount of power to make sound.