In economic terms, it is beneficial for companies to "commoditize their complements." If flights to Miami become cheaper, hotels there can raise their rates. When web browsers became free and ubiquitous, it allowed web applications to prosper on the new influx of people with access to the internet. Google built an empire on this principle.
GitHub commoditizes (or more accurately, pressures costs lower to commoditization for) several complements that would otherwise reduce the profitability and viability of software based businesses. It provides widespread, free access to software as a hosting provider of open source software. It also provides widespread, comparatively low cost access to proven software talent as a sort of professional social network. As the costs associated with one part of an industry fall, the company that causes that decline and the companies that are complementary to those costs benefit greatly.
It is hard to start a new company offering a product for a market inefficiency that has been thoroughly solved for most use cases by commoditized, open source software. However, companies with a complementary relationship to that software will benefit greatly, either because they will have a wider market potential through new customers or because their own costs will drop precipitously. Similarly, developers commoditize themselves and their own costs to a company if they cease differentiating themselves or make it easier to find a supply of them. As an exercise, who do you think is better off in the video game industry - video game studios who make games with a de facto maximum price of $60 (ignoring DLC and in-game purchases), or Microsoft and Sony, who can charge for access to all of those games on an initial (hardware) and monthly (network) basis? Do you think it is in Apple's favor as a company that sells expensive hardware to have developers on their platform charge more or less for their software?
Anyway, my main point was acknowledging that, yes, GitHub makes a lot of other software companies easier through this principle, but no, I don't really agree that venture capitalists have this as an explicit investment strategy. That attributes to them spectacular market forecasting ability and there is a much simpler explanation, which is that they actually believe the company will do well.