LLVM/Clang can target many different platforms besides windows and it is open source so it is sponsored by many large companies including Google Apple etc.
Google use it for Chrome for example (which includes windows) and have engineers working full time to meet their needs.
Intel, ARM, AMD and co have upstreamed many optimisations into LLVM also.
If you have a bug it probably will be resolved faster. Instead of waiting months for the next Visual Studio Release etc
In general, LLVM because of the intermediate bytecode language and well made libraries like libclang enables new kind of tooling.
Microsoft actually use LLVM themselves in many places and have a bunch of open source projects that use it, which is great
The would actually probably be better off retiring MSVC's c++ compiler and focus on clang along with everyone else because other companies are basically paying for the engineering for their platform with clang.
Unlike gcc the license for llvm/clang is more liberal so if they want private internal tooling they can still do that also.