The ability to view the source provides an understanding of the engine that can be valuable to wringing the most performance possible out of the engine and simplify debugging of the title. Source code also allows customization if there are particular, unusual requirements for a title. Finally, there are two primary pricing models, royalty and royalty-free. This model is very appropriate for a startup organization that has little cash, or for a very special-purpose title that is likely not to sell many copies. The key advantage is that there is no additional impact on profits should the title become a big hit.
Licenses can be for a single title, or for multiple titles developed at a specific location.
While we have focused primarily on the economic issues, there are several other non-technical issues that often lead to the decision to build rather than buy. The most common objection is that the engineering team can build the engine themselves. A savvy developer can certainly turn out an excellent game engine that is very specific to the needs of the title under development. There is also often a concern that lack of control of the underlying engine will result in either a game that looks like others or code that cannot be readily and quickly debugged at crunch time. A general-purpose engine provided by a company committed to game engine products does not have these issues.