These functions have polymorphic return type, since they do not return. Naively, this doesn't mix well with variadic functions: if you define, say,
let f fmt = ksprintf (fun s -> failwith s) fmt
then you find that f "%d" : int -> 'a, as you'd expect, and f "%d" 7 : 'a. The
problem with this is that 'a unifies with (say) int -> 'b, so f "%d" 7 4 is not
a type error -- the 4 is simply ignored.
To mitigate this problem, these functions all take a final unit parameter. These rarely arise as formatting positional parameters (they can do with e.g. "%a", but not in a useful way) so they serve as an effective signpost for "end of formatting arguments".