Not having to constantly make minor adjustments to throttle position to maintain the correct speed is a big bonus for me, both in traffic and out of it. I find I can focus much more on watching my surroundings, and whats happening 500m down the road, rather than constantly looking at the speedometer. This also makes a big difference to how tired I feel after long trips. The same applies to the Lateral Control part of Autopilot, although I use that less.

(To be fair, many other cars provide adaptive cruise control too, but Tesla's is the best I've tried so far).