Why does `Writer.tell` return `Writer[T, Unit]` ?

[atais]

Hello.

I find it not handy at all. Let's say all my logic is based on a Writer[List[MappingError], T]

and now I can (sometimes) nicely compose it with value:

val foo: Writer[List[MappingError], Foo] = Writer.value(fooValue)

but now with tell:

val foo: Writer[List[MappingError], Unit] = Writer.tell(List(someError))

how is the Unit ever useful? I would expect it to bind to generic T that composes with the rest of my writer. Or there is some way to transform it, that I dont know of :-)?
Thanks!