Some users are accustomed to command completion functionality.
That's why in WinEdt's default settings a few such responses are defined but not enabled. In the Edit control next to the enabling check-box you can also specify a mode filter, thus to selectively enable automatic response only for certain types of documents. What you see in the default settings is only the tip of the iceberg. Another set of active strings are delimiters. WinEdt is by default set up to perform auto-matching of brackets while they are being typed.