The .NET community largely grew up having a "source of truth" for best practice, architecture, etc. Microsoft shaped not just the language, but the culture around the language, having vast influence on how people learnt the language, the processes on how they worked and naturally used this to push MS technologies. MS tried to own the whole stack, and thus they had libraries and frameworks for much of what you would try and do with .NET.

While competing against other companies this strategy worked as MS were larger and able to have a more complete solution. This was enabled by their huge share of desktop computing with Windows. However as the shift to Web and Mobile came, the number of platforms grew (especially outside of MS controlled ones), and the centrally planned .NET experience was breaking down. MS's centrally planned competitors threw in the towel or had gone bankrupt. Java was opened, Sun was out of business and the smaller software development offerings had long since tried to incorporate open source, or had switched to growing only through milking the existing customer base.

Microsoft saw the change coming, as well as the move to the cloud, and was able to see the cost involved with competing against open source, and started their shift to open source. The foundation is their attempt to both reduce their relative investment, by having users start taking on some of the development while controlling the core of the language. Keep in mind this was the time .Net was becomming cross platform, with linux becoming a major hosting option.

Due to these origins it has lead to the foundation being in a central position in the .Net ecosystem. As such if your library is part of the foundation, its likely to be THE standard solution in the .NET space. So the advantage you get is you get to participate, and if your project joins, it becomes the de-facto standard for .NET. That credibility is likely worth a bit for consultants and definitely paints you as an expert in that field. The downside is you cede control. I am not arguing that it is worth it for most people / projects, however I can understand why people want to be involved.