For a site with a handful of pages and a single author who's happy to use django.contrib.admin I agree with you that plain DIY Django will probably get you there more quickly. But Wagtail (and other Django CMSs) gives you a lot more than that:

- a fast, intuitive content authoring experience

- preview / submit / moderate / publish workflow

- a UI which scales to very large numbers of authors, pages, images

- powerful full-text search in the front- and back-ends

- strong support for image handling (e.g. animated gifs via Wand) and media embedding (OEmbed or Embedly)

As an agency, these points are all pretty much requirements for the sites we build. We couldn't find anything else which gave us these as well as allowing us to build models and templates in plain Django, and we had a client - the Royal College of Art - who agreed to let us open source Wagtail on the back of what we developed for them.