As Simon Peyton-Jones put it[0], Haskell is a language where people work as well as play. Sometimes there's an obvious practical use, sometimes it's not so obvious or may not exist at all. In this case, I as a Haskeller don't see a good use of "forbidden types", however, type families have many practical uses, see [1].

In the same lecture, SPJ elaborates on how the usefulness of typeclasses weren't apparent in the beginning only to be discovered more later, in part through people playing.

[0] A History of Haskell

[1] https://www.microsoft.com/en-us/research/wp-content/uploads/...