What exactly is `span_p<>`?

The Core Guidelines specify span_p<> like this:

• span_p<T> // {p, predicate} [p:q) where q is the first element for which predicate(*p) is true

But why would this be a dedicated type rather than simply a span<> algorithm?

template <typename T, typename PredT>
span<T> take_until(span<T> s, PredT&& pred)
{
    ...
}

Perhaps I'm misunderstanding the intention of span_p<>? Could somebody elaborate?