1. Coinbase raises money from the IPO.
2. Coinbase takes some of that money and gives it to Grayscale.
3. Grayscale uses the money to buy cryptocurrency... On Coinbase!
Not only do Coinbase shares appreciate from the increased value of assets on their balance sheet, but they earn some of their money back from fees, too. Just make sure that steps #1 and #2 have some delay, after any lock-up period.
And since Grayscale can only divest from its funds for fees and doesn't allow redemption, more cryptocurrency tokens get "locked" in the funds indefinitely, unable to affect the spot price.
Traditional stock market buys tons of coinbase cash -> coinbase uses said cash to market-buy BTC with bots driving the price up -> BTC/their crypto of choice is now worth far more.
If the market follows their run they end up making far more than they reinvested.