I'm going to be ignoring Richard Seabrook's amazing accomplishment a bit to talk about the benefit of opening up intellectual properties.

In this instance Steam was well within their right to not allow Richard to sell his game as a stand alone product. Yes it could have been released as a Half Life 2 mod but the publicity that the company can get from allowing content creators to build freely is much more beneficial.

The same thing applies to the open source community in software. Any software product that isn't adaptable rarely sees any attention. At the minute I'm able to set up my personal website for free using Microsoft Azure and Visual Studio because they know that if developers are using their products, then they'll be more likely to have companies using them. Companies that will pay for licenses. 

Its not just software and video games though. Recently D&D creators Wizards of the Coast released the reigns on publishable content slightly and has seen an influx of people not only creating new scenarios but buying their own books. For a while a lot of books that use the FATE system have been self contained but that hasn't stopped the sales of their core books.

If correctly marketed, a free core product and the ability to modify openly can have a large affect on your other products being noticed.