The advantage of using micromorts is that they're units that can be added and directly compared. As a populace, we tend to be rather bad at doing fractions and percents - things like mpg - going from 34 to 50 mpg is less of a savings than having a vehicle go from 18 to 28 mpg for the same distance traveled.
Consider the problem: you must either do X (1/200 chance of death) or YZ (Y has 1/300 chance of death, Z has 1/300 chance of death).
Reworded this becomes: you must either do X (5 micromorts) or YZ (Y is 3.3 micromorts, Z is 3.3 micromorts). And now you've got things that are easier to compare and work with in a scale that we can think about. Most people are doing things on the scale of 0 to 20 micromorts. The more adventurous are doing things that get up to 100 or so. The thrill seekers go up from there.
This is also a concept in insurance - https://www.insurancethoughtleadership.com/six-things-commen...
Cambridge University - Professor Risk https://youtu.be/a1PtQ67urG4
A previous discussion on it from 2013 https://news.ycombinator.com/item?id=6266462