For their 30% ...
Google provide payment system (for app purchase and in game), which makes life easier for a small dev as the hassle about complex PCI DSS requirements is removed, just use their APIs & follow their simple rules
Google scan app for malware, reassuring dev and end user.
Google freely "hosts" your app for people to download.
Google supports (via Google play games) in game "achievements / rankings" saving game devs that hassle as easy to use their APAs
Google provide free app review system via ppplay store
Google do some marketing for you e.g. people who used X also downloaded Y (where Y is your app)
If your app gets good reviews it will rise up the rankings for its sector in the play store,increasing its visibility to users ("free" marketing based on reviews).
For a huge company then worthwhile doing it themselves, but for small app maker the "gouge" can be worthwhile given the benefits.
Full disclosure, I have had app on the play store...
It was free (no ads either) and so cost Google money to host it. Lots of apps on there are free and ad free : Not everyone releases apps for profit but because they thought others may find it useful / interesting, or (in many cases) / to provide mobile interaction with their main product (e.g. don't like our website on mobile?, use our app instead)