New material to improve on the robustness of the bezel of these watches. Basically the older aluminium bezels were prone to fading, scratching, breaking etc and long term usage by divers or people who actually use the watches as the tool watches that they are, were not guaranteed. So, the ceramic bezel came about, the lm bezel came about. The sapphire crystal covered bezel came about.
You can get pretty technical about lm and it's qualities and it's manufacturing process. But do you want to go there?