It can be challenging to fully appreciate all of the possible threat scenarios, especially if you don't have a specific threat model that you are attempting to mitigate. I was in the same boat when I first read the GOS documentation and the posts here and on reddit (long before actually using GrapheneOS).
For many users, there is no issue at all with running the sandboxed Google Play in their one and only profile. In fact, I would wager that many if not most GOS users do this already.
Some of us however, would rather not use Play at all, or at the very least some might wish to limit its interaction to a specific set of apps. I commented about this just a few minutes ago on another post if you're interested. As I mention there, Play Services and Store apps are widely used by apps as a conduit for communication, telemetry, advertising, and other services, so for some it makes sense to isolate Play from the rest of their apps if they wish to avoid those things. IPC allows apps to share information with each other, as long as there is mutual consent between the apps, which would be by design of the developer(s) themselves.
With the exception of using profiles to separate apps and data, there is nothing the user can do to control this behaviour. IPC is a fundamental part of the Android OS and is often used to perform common tasks between apps. However, it can also be used to exchange telemetry, metadata and other information between apps.
In another scenario, a user may have two separate social media apps with two separate pseudonyms. IPC may allow one of these apps to share information to the other (again, with mutual consent) which could possibly reveal that their pseudonyms are from a single source.
These are just a couple of many examples as to why someone might choose profile isolation.