I imagine there are different motivations for different people.
Some people may just want to use rust to re-implement something that exists in order to hone their skills or just for fun. Having a reference implementation makes that more productive.
Other people may think that it would simply be better if the project were implemented in rust. This is the case for some C projects, where a lot of control is still necessary but the project would benefit from the safety and productivity gains of rust. See the example of QEMU yesterday. It is an absolute pain in the ass to do things well in C. Correct programs end up being heaps of boilerplate and tests. I'd love to escape C but haven't been able to until Rust.
Maybe some people just want to showcase that Rust can do the thing. I think that's fine. It might help in adoption. It might cause better libraries to be created that are actually used.
For me, rust is the first real competitor in the systems programming market. There are hundreds of languages for application programming. I prefer Haskell, Java and JS for those but I've never found a replacement for C in the low-level domain. C++ just never cut it for me and the alternatives never really caught on. For me, Rust brings a lot of what we learned from Haskell and ML to C without sacrificing the low-level control. If I wanted to get better at Rust, I'd probably also implement something that exists just so I could check my work. If I was to re-implement some of the low-level projects that I've worked on, I'd also probably do that in Rust.
I'm getting really tired of comments like this though. Why does it bother you so much? Are you trying to communicate that you'd prefer this person be doing something else with their time? What does that even mean?