If you want to look at it from an academic perspective it tells the compiler that it should be a relevant[1] type. The idea is that you can make the compiler f

If you want to look at it from an academic perspective it tells the compiler that it should be a relevant[1] type. The idea is that you can make the compiler force the use of the value at least once. This is great for error or result types so that you can require the caller of your code actually check for an error or valid result, this way you can't forget to do it (if you do it's a compile error and you never get to run the code). [1] https://en.wikipedia.org/wiki/Substructural_type_system#Different_substructural_type_systems https://en.wikipedia.org/wiki/Substructural_type_system#Diff...