Several designs I've seen have a load of vias underneath high frequency or analog ICs. For instance, this is a part of a PCB design for a DDS unit:
Design from http://rudius.net/oz2m/ngnb/dds.htm
Note the grid of vias underneath the AD9912. This is not a particularly good example, as the AD9912 has an exposed analog ground pad and the ton of vias is presumably just for a low impedance return path to the ground plane, but I'm sure I've seen this in lots of other places too, some of which don't have exposed pads underneath.
Is this perhaps to effectively move the ground plane closer to the sensitive IC, reducing impedance further?
Apologies if this has been asked before but I couldn't find it.