Besides the main compiler part, the project comes bundled with an interop layer, which allows you to easily call C# code from Rust and vice versa.
Besides that, the interop layer (which is not yet finished) will also contain support for defining .NET classes from Rust code.
The main use case I imagine is using Rust libraries from .NET. This can decrease the GC usage, while still allowing you to easily hold references to managed classes.
This is also what the interop layer is geared towards: I want people to be able to quickly write managed wrappers around Rust crates, and for the Rust compiler to verify this glue code is safe, both on the Rust and .NET side. A lot of this is WIP, but in the future I will be able to fully verify the safety of such glue code.
Having a pure-il assembly also enables one executable to be used across all targets .NET supports. Adding another pure-IL dependency can be easier than adding one that ships with an unmanaged library.
The Rust to .NET translation is also very high-level: it preserves things like argument and variable names, types, field names, etc. You will also be able to catch a Rust unwind in C# code (as a normal exception) and Rust code will be able to catch .NET exceptions.
Overall, the goal is to provide more performant .NET libraries, which can be used without even knowing that they are written in Rust. This way, the cost of moving existing codebases to such libraries will be much smaller.
Please let me know if you have any other questions, I will be more than happy to answer them :).