I think such combat robots might be usefull. Let's consider why.
Firstly, today we're using flying combat drones that are really effecient to supplant conventional aircrafts. They can sustain much higher speed, because you don't have to care about a human suffering from g forces inside your drone. Nonetheless, even if they're pretty efficient they have the same problem all aircrafts got. When you're fighting in higly dense area, like forests, mountains, towns, these sort of drones tends to be less adapted.
Secondly, you might think then "Eyh let's use flying drones with propellers, these would act like helicopters and works perfectly". Helicopters are far more adapted to urban environment for sure. Nonetheless that's the kind of hard to maintain equipement, I can't find my source anymore but I read somewhere on world building SE that helicopters had a ton of drawbacks preventing armies from having hundreds of thousands of these instead of using trucks. On top of that, even with no drawbacks, you're not going to fight inside buildings with such drones. And you definitely can't do it either in forests.
So then another solution is using drones with wheels/tracks or perhaps many legs, but not humanoid shaped with two legs and two arms. Wheels and tracks ? Not adapted to difficult terrains too. Many legs ? Means more cost, will be more stable and more efficient than two legs robots. At least, robots with legs would have that advantages animals and humans got to move on almost every kind of terrain.
But then why giving arms/heads/eveyrthing that defines a human shape to a robot ? Well perhaps for military it's no use at first, in combat situation a robot do not need arms, it can have turret mounted weapons most likely and accessories to supplant arms, it does not need a head either, or not the same one we human got. But outside of fighting ? Don't your drones have any other goal than just to fight ? A soldier does not fight each second of his life. He need to do things that could require arms, like building stuff (fortifications most likely), or transporting supplies, or doing some maintenance tasks, even help the civilians (don't forget today's armies are often used to provide assistance to civilians in case of natural disaster)s.
Last point for me would be : Civilian use and war economy. If the army is producing drones for it's own purposes, it is very likely the civilian industry will produce drones to. If they are enough advanced, people would really like to have a robot with which they can talk and live. And I guess everyone likes more to live with something that mimics living beings (so cats or dogs, or even human-shaped robots) than with weirds robots looking like a tank.
Would you like to talk to a tank in your dinning room ? If yes, then you're really hardcore sir ! But anyway this is not the point.
If drones are produced by the civilian economy, esthetics factors (like humanoid shaped robots) needs to be taken into account for customers, thus having humanoid like robots makes sense. And when war will break out, all the robots produced by the civilian industry would make perfect second hand drones to use as military stuff. They'll just need to be refitted a little, like adding a new shape with a bit of armor, creating some programms to teach them how to fire, and give them small fire arms and here you are, cheap robots fighters coming from the civilian industry without any need to heavily convert your civilian factories into military factories. Easy isn't it ? Even easier than converting cars factories into tank factories.
It would most likely give a great advantage in the case of a total war, to have at once a full stock of ready-to-be-converted robots. The best is, before any war breaks, the country using such method would be considered pretty weak "Eyh look at them they have such a small army, they are no threat to us." And then the war breaks, and millions of civilians robots are turned into fellow fearless soldiers, ready to fight for the mother/father - land.