Yes, that's quite fast. The files are kept in a version control system so you update from that first as well.

The use cases for images would be: delivery of applications to users, or (internally) delivery of some fixed set of underlying code that is used as a substrate for development but that wouldn't normally be changed by developers.

Normally, the developer has an image running all day, and does development and testing in it, but (unless needed for additional testing) doesn't save that image as a new binary that others can run.