Why put vias underneath an IC?
With non-microwave devices, it's usually to conduct heat down into the copper planes within the board, or through to the heatsink attached to the back of it. This particular DDS device is fairly watty, so needs more cooling than the top surface and the leads can provide. One of the major manufacturers has registered the term 'power pad' for the exposed bit of metal under the IC.

Many microwave devices also have exposed pads on the back, but these are usually for grounding as well, where a good microwave match needs a very close ground.

Some surface mount power FETs have an exposed pad, and this can be intended to take tens of Amps (yes, scary), which needs a good array of vias down to multiple wide tracks.

Beware, almost all ICs with exposed pads are designed for them to be connected to ground. Just a very few are not. My team have been bitten twice by components, one amplifier, one analogue switch, where the exposed pad went to the -ve rail instead of ground. Much wailing and gnashing of teeth while we waited for the corrected board to get made. Check twice before you lay your board out.