Since OP3 maps are going to be 3D, it seems like a robodigger would have a great deal of potential.
For one thing, it would indeed be a great way to pass previously impassable terrain. Doubtless the tunneling would be a non-trivial process probably requiring a significant amount of time.
Terrain would vary underground too, some types of rock would be prone to cave-ins, others would be too fragile to dig through at all. Heat from volcanic phenomena would create impassable areas as well. On the other hand, some areas might be so stable that the only thing that could cave them in would be an explosion.
Natural caverns might also exist (depending on planet) that would either make tunneling easier if they were in the right place, or harder if they happened to be in the wrong place.
For war efforts, I can see people trying to break bottlenecks with a tunnel, but it would not be an easy task. Tunnels are all too easy to cave in, and I can see defenses being devised for the express purpose of countering diggers (robo-moles rigged with explosives anyone? Certain guard posts could be upgraded to contain a supply of them that would prevent enemy tunneling within a certain underground radius of said post).
EDIT: Now that I think about it, even detonating some Starflares (or equivalent OP3 vehicle) above an enemy tunnel might cause enough seismic activity to collapse it. There should be no excuse but laziness for letting an enemy tunnel under your base.
If an enemy were foolish enough not to defend against underground attacks, then I think a particularly cool idea would be to send supernova-type vehicles into a tunnel deep under the enemy colony and detonate them, causing a minor earthquake above (but collapsing the tunnel of course).
Or maybe dig underneath their mine and cut off their ore supply?
On a non-combat front, I can envision robodiggers being used in mining or resource transport.
Here is how I think mining will go in OP3: Since it is 3d, resource veins will be located a certain ways underground. Robominers must get within a certain distance of those resources, since logically the robomoles would only have a limited range from the miner.
Now imagine there is a large deposit of resources below some very inhospitable terrain. It would be infeasible to move the robominer close enough, so instead you use a robodigger to dig a long slanting tunnel below the terrain, and then deploy a robominer at the bottom of that tunnel.
It would be a difficult way to mine, but it might be worthwhile for particularly valuable deposits.
While we are at it, why not have a robodigger hollow out an even larger hole in the ground, and then construct a building in it. An underground building would be protected from disasters and attacks (at least until the enemy brings in their own diggers, or severs some tube connections).
I can even see custom maps being made with so many disasters that the only safe place to build is underground. Such a map would present some unique challenges.
EDIT:I know this topic is not supposed to be about building underground, but come on, that is a logical extension of a digger. Certainly building underground would not be the best choice, or even a good choice for many situations, but there could certainly be reasons for it. It would just have to have it's own set of advantages and perils.
I think the peacetime uses of a robodigger would be significant, even if it does not prove valuable as a military tool. I don't know to what extent war will dominate OP3, but in the event that there are opportunities to develop a colony that is not the focus of a massive war effort, tunneling may prove essential.
If modern society on earth is anything to go by, then there are a whole lot of reasons to dig underground.
