You pretty much always do a late-stage private round shortly before an IPO, that is the standard. The goal of the late-stage funding round is to give a better idea of how much capital can be raised by the IPO. It helps reduce uncertainty about expectations of what the company is worth before going public.

Pump up the valuation baby.

Price setting.

IPO isn't really about "raising money for the company" any longer, unless one means raising the money in their wallets so they can take the money and run.

Conspiratorially, it seems like a shotgun attempt at undermining the supposed OpenAI IPO later this year.

Also filing an S-1 doesn't actually indicate that they intend to go public "immediately," it just gives them the option to go public (probably in the near future).