Today's train systems use sophisticated electronics that are expensive and can be sensitive to things like electrical shorts and derailments (which are inevitable). The TVS suppressors help protect those sensitive electronics. They can degrade over time when they are called into play and do their job, so some people replace them every few years. IMHO they're easy to install and cheap insurance - like fast acting fuses.
If you do a search on the Forum, you'll find a more technical breakdown of how they work.