[lmm]

I had a generic report class that essentially fetched a bunch of database rows, did some stuff for each row, and then combined the results together into a report. (This was in Scala, I know Haskell doesn't have classes, but presumably similar situations can happen)

For one client, we needed to accumulate some extra statistics for each. For another, we needed to call their web API (so async I/O) to get some of the data used in the report. By making the generic superclass use a generic Applicative type, we could keep the report business logic clear and allow the client-specific subclasses to do these client-specific things and have them compose the right way.

Wanting custom applicative types is rarer than using a standard one, but it can be a good way to represent any kind of "secondary effect" or "secondary requirement" that your functions might have. E.g. "requires this kind of authorisation" or "must happen in a database transaction". But a lot of the time you can implement custom things using reader/writer/state, or free, rather than having to write a completely from-scratch applicative.