but the executable that is built with these optimizations is running on people's computers, not as a backend on google servers.. or am I missing something?
Perhaps I didn't understand the question. I thought the question was about why anyone would save a lot of money from software going "only" 10% faster. Most large datacenter operators will get unreasonably excited about even 1%. But you're probably right that Google itself will not directly benefit from making Chrome faster on macOS.