Growing pains. At a recent OPFOR exercise, unmanned vehicles were "blinded" by adversary's electronic warfare systems. The design team behind the unmanned systems vowed to learn from this "painful" experience and make improvements.
My guess would be to scout enemy position ahead of the soldiers. Then again, robot dogs are really noisy so maybe it is more of a distraction. Supposedly those could be fitted with smoke grenade/flashbang launchers.
They can use stairs, which is good for urban warfare.
A wheeled/tracked vehicle designed to use stairs may have to be bigger, heavier and ultimately more expensive.
Tiny flying drones could also navigate over stairways, but those may have to be small to do that and thus not able to use half as decent sensor packages as a dog sized platform could.