It is the modern version of a compiler construction toolkit, which by the way isn't a new idea, even though so many think LLVM was the first one,

One example from 1980s, there are others to pick from as example,

https://en.wikipedia.org/wiki/Amsterdam_Compiler_Kit

So naturally a way to quickly reach the stage where a compiler is avaible for a brand new language, without having to write all compiler stages by hand.

A kind of middleware for writing compilers, if you wish.

MLIR is part of LLVM tooling, is the evolution of LLVM IR.

Mojo uses mlir for high-level ir and most optimizations but codegen is still done with llvm, albeit in unconventional manner.