If you were new to Photoshop and I presented its entire interface immediately, you might have difficulty taking a first step. But you might feel more comfortable if you began in a “welcome” workspace with most of the panels and tools hidden. Then you could reveal each piece of the interface as you discovered that you needed, or in response to your curiosity about something.
Similarly, when explaining a new concept, it’s often helpful to begin with a constrained version of the problem. For example, when explaining how to make and balance vinaigrette, you might initially focus on a single oil/acid pair.
In video games, players often begin with a small set of abilities, or in an environment where some types of challenges never appear. This makes the game more tenable at first.
Constraints are one way to construct fine-grained task progressions (see Fine-grained task progressions as cognitive scaffolding), but they don’t have to be used this way. For example, an aspiring photographer might constructively spend an entire year shooting with a fixed aperture, focal length, and ISO, adjusting only the shutter speed.
Related: Constraints often breed creativity