Programming Phoenix 1.4, Writing Queries with Keyword Syntax

In Chapter 7 - Diving Deeper into Ecto Queries - Writing Queries with Keyword Syntax, we have:

Repo.one from u in User,
         select: count(u.id),
         where: ilike(u.username, "j%") or
                ilike(u.username, "c%")

Is there any reason in particular we are using count(u.id) rather than count(u)?