Right now, most semiconductor companies who make an SoC for whatever purpose, will often license designs for common things like a DDR controller or a PCIe interface. But in literally every architecture meeting I've ever sat in ever, the senior manager bigwig always asks: "has this design been tested in silicon yet?" And for a very good reason! No one wants to make a chip only to find out that there's a show stopping errata on the new bleeding edge Ethernet controller design. So the architecture team inevitably settles on the current generation IO protocol, which turns into previous generation when the chip is product ready.
But if your IO controller controller is instead on a separate chiplet, that risk of show stopping errata becomes less impactful (at increased the cost of packaging.) The vendor can respin, or you can choose a different vendor all together. It also allows for greater flexibility in vendor selection and SKU offerings, as well as yield improvements. But I think those upsides are a bit more obvious.
Unfortunately, it could make the business of selling IPs like memory and IO controllers that much more cutthroat, since the cost of changing vendors dramatically decreases. But then again maybe not, since a respin would barely (directly) cost the customer any money.