Set Structure Member is a Quartz Composer modifier patch and can be used to do two things:

1. Create a structure from scratch or
2. Modify an existing structure.

Quite a few built-in patches in Quartz Composer work on structures or deliver structures as output. There are also patches to get data out of structures, e.g., Structure Key Member. However, we felt it a limitation that there is no way to create or (programmatically) modify a structure inside Quartz Composer compositions. This is why we created a custom patch for doing exactly that. Look at the bundled demo composition for a simple example.