It's basically what allows you to run your own scripts or scripts from RSCA on the neXus client. Normally, you would have to pay a monthly fee to develop, test, and run your scripts (at any time of the day), or wait till weekends when iBot neXus was enabled.
This was done to encourage scripters to develop for neXus rather than other clients such as RSBuddy and RSBot, as it was previously such that you would have to pay before you could even start writing scripts for their API.