If I had to guess, I'd say that it's often better because picking a few random numbers leads to actually thinking about the decision for at least half a minute.

In practice, guessing at numbers and running a calculation actually serves as a quick second opinion on your original intuitive decision. If the numbers imply something far different from the decision that System 1 is offering, I don't immediately shrug and go with the numbers: I notice that I am confused, and flag this as something where I need to consider the reliability both of the calculation and of my basic intuition. If the calculation checks out with my original intuition, then I simply go for it.

Basically, a heuristic utility calculation is a cheap error flag which pops up more often when my intuitions are out of step with reality than when they're in step with reality. That makes it incredibly valuable.

of which figures are chosen and how the are used.

On first pass, I read this as "which figures are chosen and how the arse is used". That seemed oddly appropriate.

I figure it works better about 80% of the time, so I'm going to go with it.

There's some good discussion in Thinking, Fast and Slow about when intuition works well.