The mentors provide excellent feedback on what you should focus on at any given time, and are great at troubleshooting problems. In addition to that, they have a great network (the YC alumni which includes some self-driving car companies as well as investors) who may be able to connect you directly to the companies you want to work with.

One of the hardest things for people that have never started a company before is to understand just how hard it will be. Having people on your side can make all the difference, if only to improve morale and help you maintain you sanity.