Not an expert, but I am a power user of fuzzers.

The problem is that the space of invalid inputs is far larger than the space of valid inputs. Sometimes orders of magnitude larger, say billions or more invalid inputs to one valid input.

Naive fuzzing will hit so many error cases that it will hardly produce a valid input. For the ratio that I mentioned, you might run a fuzzer for a billion runs and only get one valid input in the bunch.

Using a custom mutator and returning a dummy value will give the fuzzer a starting point from a valid input and makes generating other valid inputs more likely.

For my part, I prefer to use custom mutators to generate valid test cases most of the time, but I want some invalid inputs because error handling is where most bugs are.