I carry one in each car because I want to know more about why the yellow light is on. Then I can decide if it's serious or not.

For someone who can do some of the work on the car it's a basic diagnostic tool. But the tool doesn't necessarily tell you what to change. It is only giving you strong hints about what's wrong. If your car systems vocabulary isn't strong enough to recognize what system it's referring to then it wouldn't be useful.

I think mine are Equus brand - slightly over $100 at the time.

(I've also made friends at work reading coworker's codes and sometimes clearing them. Can be better than putting a $5 bill on their desk when a favor is needed in the future :-)