It is my understanding that when using stack to compile a project, no version constraints for dependencies should go in the .cabal (or package.yaml) files, because the resolver picks specific versions for you. This includes the GHC version and its base library. However, when creating a new project with stack new, it automatically adds a version constraint to the dependency to base.
Excerpt of auto-generated package.yaml
dependencies:
- base >= 4.7 && < 5
Why is that?
I don't know the canonical answer. But here's one reason why it might be nice.
For what is currently the only realistic implementation of Haskell, namely, GHC, the base version and the compiler version are inextricably linked. This means that suitable base constraints also communicate which version of the compiler is intended to be used.
That latter piece is interesting information to know about a package at a glance.
Now, it's also true that stack resolvers and GHC versions are inextricably linked. So you might think that information is already available. BUT currently, Hackage (the place that most Haskell packages get hosted for use by others) displays a bunch of information taken from cabal files, but no information taken from stack files. So if you want the information about GHC version to be conveniently available from the autogenerated Hackage summary of the package, this is one way to easily and automatically do that.