We have faster iteration than upstream distros. More flexible. More tuned. I won't give exact details but 1% CPU gives extremely significant monetary savings, and there are at least 15% savings from static linking, PGO, LTO, more appropriate `-march`, more appropriate CPU security sharing considerations, ...

Billions of dollars per year, in essentially electricity and required systems savings (considering the scale of serving 3 billion users a day).

Also devs get access to the latest compilers, language levels, and libs, completely independent of distros, who have a more general compat issue to contend with. Considering there are about 30k tech in Meta this value also multiplys up.