having devDependencies gives you the opportunity to simplify the steps that drive you from the source files (a git clone of the project) to the production ready app
when you don't need to make changes and (develop) the application, you could just run
bower install --production
or
npm install --production
they work the same
bower install options
-F, --force-latest: Force latest version on conflict
-p, --production: Do not install project devDependencies
-S, --save: Save installed packages into the project’s bower.json dependencies
-D, --save-dev: Save installed packages into the project’s bower.json devDependencies
-E, --save-exact: Configure installed packages with an exact version rather than semver
bower documentation
npm install options
By default, npm install will install all modules listed as
dependencies. With the --production flag (or when the NODE_ENV
environment variable is set to production), npm will not install
modules listed in devDependencies.
npm documentation
This way you take less time to ship the app and don't waste bandwidth downloading stuff you won't need.
Given that, to me, the choice of listing ionic as devDependecy is a poor one: it implies that I could take advantage of this choice to get ready the app for execution this way:
git clone my-project
git cd my-project
npm install --production # ionic not installed here
ionic state restore
ionic build ios
Now, if you ignore the content of /lib folder in your sources, this should not work, and if it works because the ionic-cli does some more checks to save your ass, I think this is unclear.