Raw performance is great, and it’s easy to salivate over that. (Or be concerned that you're getting a 5% bump at the cost of no x86 virtualization.)
But I think Apple is doing this for a different performance metric: battery life in the MacBook Air.
Apple doesn't really break out what model line sells what, but it's safe to assume the Air is the big seller, followed by MacBooks Pro, then the iMac, then the mini, and then a rounding error of Mac Pro. For consumer machines, it’s not that compelling to say “it compiles 20% faster”. It is really compelling to say “Apple introduces MacBook Air, powered by Apple Silicon, with 12 hours of battery life, and it never gets hot.”