While Go doesn't directly generate any revenue, it's probably not without reason that Google is rewriting some important pieces (dl.google.com for example) in Go. I can imagine their engineers being much more expressive in Go than in C++, causing them to spend less time on writing the code as well as resulting in simpler code (easier to comprehend and maintain, so less time/money spent on maintenance). This doesn't generate revenue but it does optimize development processes, which could indirectly save them some money.