As FastAPI is actually Starlette underneath, with a layer of several tools on top, you can use Starlette's Request object directly when you need to.
It would also mean that if you get data from the Request object directly (for example, read the body) it won't be validated, converted or documented (with OpenAPI, for the automatic API user interface) by FastAPI.
Although any other parameter declared normally (for example, the body with a Pydantic model) would still be validated, converted, annotated, etc.
But there are specific cases where it's useful to get the Request object.