Here is the reason I think an instructor is good: My line in the corners, esp the tight ones, was completely off. So Elden and Jim with Chicane took the time to take me through them multiple times in Pueblo. It took me many tries before I finally felt like I somewhat got it. Plus my body positioning was wrong.
Additionally, they trained us on the basics of tracks, i.e. flags, and the do's and dont's of tracks, which I was not familiar with. Chicane trackday isn't just track day. Even track days can be dangerous if you have ONE person out there who doesn't know what they are doing!