It helps to increase the alien interest into the base where it is firing from, making aliens more likely to choose this base if they decide to attack some base. Thereby it indirectly protects radar-only bases. This could be done with a base rocket launcher as well though, but since I usually have that one built externally I have an internal laser.
It doesn't do much damage per shot it seems, but it does some damage. Fire rate and precision are much higher than the rockets'. You can see the UFO's hitpoints bar only once you have that type researched.