To understand why Wizards of the Coast would make an Open Gaming License in the first place, it’s helpful to take a look at the 2002 interview with Ryan Dancey, which I strongly recommend you read. 

Inspired by “open source” coding projects and the rise of Linux, Ryan Dancey, then Vice President of Roleplaying Games at WotC, proposed creating an “abstracted” version of the core game within D&D:

The idea is to abstract the "game" inside Dungeons & Dragons and reduce it to a genre-neutral set of concepts and rules. Then, we'll layer on a thick helping of D&D-type fantasy elements, like the standard D&D classes, races, spells, and monsters.

Ryan Dancey, 2002

This would be available under a “copyleft” agreement; the opposite of copyright.

 A "copyright" is a way of restricting the rights of others to use a given work. A "copyleft" is a way of forcing everyone to allow anyone to use a given work pretty much any way they want to, and not be able to restrict those rights.

Ryan Dancey, 2002

The creation of this OGL wasn’t altruistic. It was backed by a sound business strategy that would see WotC’s gaming engine, the d20 system, become the most widely adopted RPG system in the industry. In turn, this would increase the sales of their core rulebooks, upon which other books relied.

We make more revenue and more profit from our core rulebooks than any other part of our product lines. In a sense, every other RPG product we sell other than the core rulebooks is a giant, self-financing marketing program to drive sales of those core books. At an extreme view, you could say that the core >book< of the PHB is the focus of all this activity, and in fact, the PHB is the #1 best selling, and most profitable RPG product Wizards of the Coast makes year in and year out.

The logical conclusion says that reducing the "cost" to other people to publishing [sic] and supporting the core D&D game to zero should eventually drive support for all other game systems to the lowest level possible in the market, create customer resistance to the introduction of new systems, and the result of all that "support" redirected to the D&D game will be to steadily increase the number of people who play D&D, thus driving sales of the core books. This is a feedback cycle -- the more effective the support is, the more people play D&D. The more people play D&D, the more effective the support is.

Ryan Dancey, 2002

This strategy seems downright prophetic to modern eyes. The last 20 years have seen the 3rd-party market thrive under the Open Gaming License, first driving players and creators to adopt the d20 system, then driving them to the 5th Edition D&D system after a disastrous flirtation with abandoning open gaming with 4th Edition. In 2002, Dancey laid bare the strategy for a feedback loop, and history has proven it to be sound enough to become the backbone of the industry at large.

At the core of this idea was to have as many creators as possible adopt the core concepts of D&D. This includes classic spells, items, monsters, and classes, which would be summarised in a document called the System Reference Document (‘SRD’).

The idea is to release a D20 System reference document under the Open Gaming License; essentially exposing the standard D&D mechanics, classes, races, spells, and monsters to the Open Gaming community. Anyone could use that material to develop a product using that information essentially without restrictions, including the lack of a royalty or a fee paid to Wizards of the Coast.

Ryan Dancey, 2002