The advantage is cross compilation. Gcc and friends are notoriously difficult to setup for cross compilation (so much so that some don't even bother trying and either only offer source or just compile from different machines running the target platforms, and even this can be a bit of an ordeal)

Zig uses LLVM under the hood so it does benefit from LLVM upgrades. Also, there's devils in the details. For example, the copy ellision thing is something that required deliberate implementation; it doesn't just come for free if you're naively emitting C.