The main thing is it gives the capability to adjust almost every detail, and they have a tendency to become important in embedded applications. To give one extreme example, an embedded intel board had a hardware errata which basically meant a very common sequence of instructions was unreliable, and the workaround involved patching the compiler to avoid emitting it, but then basically everything needed building with the patched compiler. Yocto lets you do that, it's even fairly easy, most traditional distros would struggle (Gentoo and nix are the other options, but I don't know how well they can do cross-compilation, which is also a big part of Yocto).
I've used it a fair amount, to build x86 on x86_64, to build arm 32 on x86_64, etc. It will also let you build x86_64 on x86_64 but for a different CPU type, so i can build packages/binaries for older systems on a newer system (like, a system with no avx-whatever can be build on a current-gen machine where the compilation goes way faster but builds binaries for the older system.)
Ah, cool. It's been a long time since I used it in anger, and it was not so hot then (yocto was I think the first system out of 3-4 I tried that actually gave me a functioning cross-toolchain, and gentoo's crossdev was one of those. This is like a decade ago though).
maybe longer than a decade, i've been cross-compiling on gentoo for at least 13 years without issue (that's 2012, for those playing along at home.) I say "at least" because i can't remember doing it prior to raspberry pi...
