Being a Google Partner has many benefits, but they revise them all the time. Google seems to protect their intellectual property heavy, so as a partner, you can display their partnership logo to your potential clients.
Also, when they roll out new products in beta, you know before the public does. As Google products go, that's a huge benefit. They provide enhanced support when you need it as well.
I also do a lot of public speaking, so the one benefit public speakers enjoy is that they let you speak at their events; a very valuable benefit to us public speakers ;)
I hope this answers the question.
The benefit is having the ability to manage more accounts easier with different interfaces.
Which is critical for some agencies.
You may also get to talk with a Google specialist from time to time.
In my experience, that's about it. If you start spending millions with them, then they will be more supportive.
Think about it like Amazon with two million sellers. So if this was a similar situation but just a different company (Google), what advantages do you think you would get? Mostly software capabilities and very little people support is my point... We can discuss in depth if you would like. I'm not sure of any decent sized inbound agencies that don't work with Google.
Does that help?