they're not exactly like the clocks on Earth (with minute hands and hour hands), but they are very similar to the clocks inside computers that keep track of time. It is very important for all of the robotic spacecraft that we send into space to know what time it is. That way we can tell them "At 11:30, you'll be flying over a really neat canyon. Take a picture" or "At 9:15, the sun will come out, so you should have plenty of electrical power to turn on the radio." Another factor in all of this is that during the months when Mars is the furthest away from the Earth, the messages that we send to the spacecraft via radio (travelling at the speed of light) take almost 20 minutes to get from earth to Mars. When the spacecraft sees the message, and generates the acknowledgement message ("I got it"), it takes another 20 minutes for that message to make it back to us controllers here on earth. This is what we call a "40 minute round-trip light-time (RTLT)." This makes it very important to have a clock on the spacecraft for coordinating events, because if we waited until 11:30 to tell the spacecraft to take a picture of the canyon "now", the message wouldn't even get to the spacecraft until 11:50 when the canyon was long gone.