This balances the airflow, so you don't get non-regenerated (or more accurately, less regenerated) airflow "somewhere else" like you do if you use a single unit.
"in thermal regenerators, the hot and cold fluids pass through the same channels in the packing, alternately, both fluids washing the same surface area. In recuperators, the hot and cold fluids pass simultaneously through different but adjacent channels."[0]
While they are indeed simpler, my understanding is that they don't really perform well. There's no humidity recovery, the CFM is too low, the cheap units don't do any active pressure balancing, you can't continuously exhaust from bathrooms/kitchens, and the lack of net flow to distribute air throughout the house means you're mostly exhausting the fresh air you just brought in. Not great.
If you're going cheap, skip the regenerator-type and the HRVs altogether and go straight to a jury-rig ERV.[1] Though again, ideally you can run flex duct (or temporarily on the interior, even just dryer hose) to pull exhaust air from the opposite side of the house, and specifically from bathroom/kitchen areas.
However, first you should test your CO2 levels and/or do a blower-door test. Most buildings aren't tight enough to need an ERV anyway!