when to use defer in use-package?

Quoting from use-package

If you aren't using :commands, :bind, :bind*, :bind-keymap, :bind-keymap*, :mode, or :interpreter (all of which imply :defer; see the docstring for use-package for a brief description of each), you can still defer loading with the :defer keyword:

But why I found many people including jwiegley himself still use :defer even it's already have :bind

• The example you linked from Wiegley's configuration uses :defer 5 (not just :defer on its own) to load the package automatically after 5 seconds. – Omar May 6 '18 at 15:07