I think the first practical mechs will be much smaller and lighter than the typical sci-fi behemoths, and they will most likely support infantry. Some of the previous users had jokingly said mechs would be used as a target, but that could seriously be part of their role on the battlefield, especially if they are unmanned. A robot isn't innately concerned about self preservation, so it can be used to draw enemy fire while providing covering fire for human troops. Mechs could also be used for mine sweeping, bomb disposal, around hazardous materials, and for lifting heavy objects. I imagine they would be more of a utility vehicle/robot than a tank, and they may be "optionally piloted" too.