Limestone is mined where oceans once existed and are remains of sea life, oysters, clams etc. You can get ground limestone at your local farm supplier like Agway. The use of a buffer like limestone will prevent the ph from going below a certain point and keep the bacteria from dying and your compost pile alive. An analogy would be, If I were living in a close environment and the waste materials that I produces are not removed. That stuff pilling up around me would eventually kill me unless I had some way of removing it or neutralizing it.
Anaerobic compost can be quite acidic.
It is a caustic substance that, in quantity, kills off masses of composting organisms and reacts with the composting materials to generate ammonia gas. I have no personal experience with lime. For all I know, lime can be added to compost in modest amounts without doing great harm. The important point is that it's a waste of lime, and unhelpful to the composting process, to adjust compost pH with lime. Lime doesn't help the composting process.