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#Diff...

It'll throw an error for

    myFunc();

but not for

    auto foo = myFunc();

if myFunc returns a mustUse type.

The official documentation[0] is cryptic as well.

[0] https://dlang.org/library/core/attribute/mustuse.html

@mustUse is a user defined attribute the compiler recognizes that forces return values it affects to be used.

e.g. if you return something that could contain an error then ignoring the return value could be disastrous.