[EricH]

There are potentially--emphasis on potentially--a number of benefits to both a smaller wheel and an 8WD individually, or as a pair.

8WD is more stable than a 6WD drop-center configuration; it has 2 wheels on each side carrying most of the weight, instead of just 1. This only works well if the center two wheels on each side are dropped; otherwise, it can turn fairly badly.

8WD can have a lower ground clearance, due to the extra wheels; this gives a lower CG, which generally means less tipping.

Smaller wheels mean less weight per wheel, and less gearing to get to a given velocity (which may mean less weight in the gearboxes). On the other hand, you do get less ground clearance, which may mean more wheels.