Most likely it’s looking for the very same thing that VAC looks for when it comes to cheats that have built in DRM that are phoning home to particular servers. Your cache gets checked to see if there is evidence of that as a lot of hacks use that particular system.
It’s been going on for years and if you’ve used PB or played any Valve games it’s doing the same thing. ESEA is incredibly more invasive as well. But it’s been proven over the years to have the cleanest community and servers by far. Of course you have to balance that against some of their past issues of abuse (and why I won’t use them anymore).