Satellites need to slow down to speed up when they perform orbital maneuvers such as transferring to a higher orbit. This process, known as the Hohmann transfer, involves the satellite first slowing down to drop into a lower, faster-moving orbit. Once it reaches a specific point in this lower orbit, it then speeds up by firing its thrusters, allowing it to transfer to a higher orbit. By leveraging the physics of orbital mechanics, this technique efficiently uses fuel to change the satellite's altitude and speed.