What is the purpose of this grid on the ground plane near a wifi antenna?
You're right, this is a ground plane, but as the antenna to the right is a PIFA, its characteristics are most influential on the antenna impedance and its radiation pattern.

This looks like a way to be flexible about antenna matching:

If after design some parameters of the board change (e.g. PCB substrate \$\varepsilon\$), or its surroundings (e.g. battery in other place, player's hand closer to antenna than originally planned), then your antenna doesn't work like designed anymore.

You'd need to "pieces" of capacitance and/or inductance to it. These small squares basically allow the factory really really late in the production process to re-tune the antenna.

So, that's a smart move to have a perfect-as-possible antenna without having to constantly respin the board just because you need a tiny shift in your Smith chart.

Instead, you just add a 0 Ω resistor and a cap (or multiple) until you found a good way of making your antenna work as desired.