I started to type an answer to this, but I realized that one of my colleagues addressed this issue quite exhaustively in his blog recently.

(Sorry to plug a blog here, but it's really a long answer and this is easier):

Part 1 Part 2

Some background: we're an ISV, and we've invested heavily in the Microsoft partner ecosystem. It helps us to gain VERY valuable contacts within Microsoft, and also helps us to meet other Microsoft-oriented ISVs, AppDevs, and SIs who may be potential partners.

How does it benefit Microsoft? Well, Microsoft has succeeded largely because people write software for their platform. They want to help partners, and they do: technical help, marketing help, sales help, you name it...