Launch rings are used to create a gravity free column extening up from the rings and in to space.
About this creation
Not all ships are capable of launching to space from a planet or even landing on one, small ships especially. The launch rings create and gravity free column extending up from the planets surface. A ship maneuvers itself into the rings. Once activated the ship needs very little inertia to launch to space. This can even be provided by the launch pad beneath the launch rings.
The gravity free column can also be used by ships not capable of atmospheric flight to land on planets. Cargo pods can also be lowered and launched from planets using the rings although this can be tricky.
Some bases are unlucky enough not to have a carrier stationed near by. These bases rely on the launch rings to get the fighters in the sky. For this reason the rings themselves may be targeted to stop defenses being deployed. The main machine, power and controls on the rings are located either underground or in another heavily shielded area. The rings are designed to hold up under a lot of fire.
Please not that these are not teleportation rings or hyperspace rings of any type. I will hopefully be creating a new moonbase layout which will included the rings.