Brand recognition in my opinion. It helps attract other deals with F500 companies. Adds to the resume. Not to mention the significant number of companies Microsoft owns a direct stake in. Looks like about 140 of them as seen on this Wikipedia page (definitely some big names) ...
So it opens up doors with companies within the Microsoft portfolio.
Maybe the most significant detail. Microsoft pretty much powers every branch of the USA government. I would venture to say all major USA gov branches have lucrative contracts with Microsoft. So that helps with getting government contracts.