Both, but mainly to keep your butt dry. FWIW, modern "chamois" is greatly superior to the actual chamois we used to use when I first started riding back in the 70s. I recently found an old pair of bike shorts from the early 80s and wore them on the trainer for an hour. While they were still more comfortable than wearing normal street shorts with underwear, the differences between them and my modern bibs were VERY noticeable.