I can imagine scraping companies are probably putting serious workloads on headless chromium instances. But that's really the only thing I can imagine
If you had a backend workload that draws 20MW and you make it 10% faster, you saved a million dollars per year in electricity alone.
Math may vary depending on cost of electricity.
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.
Every cpu cycle costs energy, turning into heat which the ac needs to remove. One computer is nothing but across all their computers it adds up.