Google is pretty useful.
It uses >15 TWh per year.
Theoretically, AI could be more useful than that.
Theoretically, in the future, it could be the same amount of useful (or much more) with substantially less power usage.
It could be a short-term crunch to pull-forward (slightly) AI advancements.
Additionally, I'm extremely skeptical they'll actually turn on this many chips using that much energy globally in a reasonable time-frame.
Saying that you're going to make that kind of investment is one thing. Actually getting the power for it is easier said than done.
VC "valuations" are already a joke. They're more like minimum valuations. If OpenAI is worth anywhere near it's current "valuations", Nvidia would be criminally negligent NOT to invest at a 90% discount (the marginal profit on their chips).