'Star-connect' pads are basically a trade off between thermal conductance and structural strength. The usual case for a pad de-laminating is for an edge to peel up. The star structure holds all the edges down, it's actually stronger than some of those other pads in the picture.
As to why you want to limit thermal conductance, that's not just confined to assembly considerations. While heat may not be an issue in a lot of designs, it can certainly be a major consideration in others. Ground planes and other copper fills are often used to stink heat off your small surface mount parts. So thinking about how much a component sinks and where could be important to a design and you can't always have the physical spacing between components that you might otherwise want.
Why that particular board uses them. Well it seems a little odd... Those are the pads for the SD slots external casing. It's clearly a surface mount design, so you would expect a re-flow construction where heating the pads isn't a problem.
Perhaps the SD slot has plastic components that aren't spec'd for the oven temperatures and so had to be attached separately? Or if it was a DIY/Hobby kit then that would also make sense. Of course someone could have just ticked the 'star-connect' box when they routed the ground connections in their EDA software...