What's the practical difference between a 16GB and 32GB iPhone? I'm looking to close a deal on an iPhone. What I am having difficulty understanding is the difference between 16GB and 32GB. When does it matter if you have 32GB and 16GB?

I understand, of course, the technical difference (free space capacity on the internal storage). As a casual smartphone user (have an Android but don't have many apps on it), and a mobile developer, I'm not sure I need 32GB. (I think my Android phone as 8GB, and I'm not using more than half.)

So when does it really matter? What are the "use-cases," if you will, for someone needing or using 32GB but not 16GB?

I'm also not planning to get a data plan, if that matters.
There's no difference besides the storage capacity.
As the iPhone does not have an option to extend the memory after purchase, you have to choose the right amount of storage right from the beginning.
You can always manage how much space you use by manually choosing which data to sync (music, photos, movies, apps).
Generally more available space gives you the freedom of not having to manage your sync settings too much.

16GB should be fine for most people, especially if they are willing to make compromises regarding sync settings.
32GB is good if your like to take lots of photos or have a large music library.
64GB is good if you record HD movies which increase in size very quickly the longer you record.