My understanding is that half-speed mastering was used to cut discrete 4 channel disc in the early 70s. It was developed so they could accurately cut 40kHz+ signals to the vinyl. It was thought that when used on normal stereo recordings that greater fidelity would be achieved, but there is some controversy over that claim. As a process it has stood the test of time, but in and of itself half-speed mastering doesn't guarantee better quality anymore than any other "remastered" claim.
