The big survey, that this links to, is to determine who is a good fit for testing other products and services, as well as providing feedback on existing services. There's nothing shady about it. Once this one is done, if you flag as a good fit for anything, they'll send an email inviting you to do another shorter survey to see if you're a good fit for that feedback group, and if so then they will ask for your feedback and compensate you for your time. Sometimes those are video chat interviews, sometimes Forms to fill out, sometimes it involves meeting with one of their product reps, etc. The format can change, but the idea is always the same. Trying to get as much high quality feedback as possible.
In addition to just the coolness of having early access to products and services (I got to use Allo and Duo all summer and chat it up with some G nerds while everyone else was just reading rumors I started

, tested Trips, and a couple that I can't name), gotten to play with devices (Pixel, Pixel XL, Pixel 2 XL, Google WiFi, Google Home, etc.) well in advance of their release (sometimes a month early, sometimes several months), I've had gift cards, swag, huge discount opportunities on the google store and the google merc store, got a free Pixel XL when I tested that 2 summers ago, got a coupon for a free 2 XL after testing that this past summer, got a free Pixel C and 40% off a Pixelbook... they're definitely willing to make sure you're well taken care of for helping them get solid feedback. The above is not typical, I got into testing stuff early through a friend at G that got things going, but I definitely enjoy giving feedback and testing products, assuming that it feels like there's someone on the other end that's actually reading that feedback. Being able to have influence, even if it's a tiny amount, on the direction of things is a cool feeling.
Not everyone who does this big survey will be invited to test products and services, and of those that are, not everyone will get early access to those things, but almost everyone who is selected and provides genuine and thorough feedback is asked again in the future.