Yes, without these pins the switch can twist. Making them a mess to align without a plate. With those extra holes not only can you use the circuit board without a plate, you can also use any pcb mount switch you may have laying around, or have bought in bulk.
As for the ground plane [0], you usually want this uniformly connected despite the grid routing of the matrix. But since there is no connection on the plastic pins you can use these holes for that instead of making a bunch of other connections.
As for the ground plane [0], you usually want this uniformly connected despite the grid routing of the matrix. But since there is no connection on the plastic pins you can use these holes for that instead of making a bunch of other connections.
[0] https://en.wikipedia.org/wiki/Ground_plane#Printed_circuit_b...