There have been various topics on device grouping and tagging (such as by location). This request is a generalization of those topics.
I think it would be useful to arbitrarily group a set of devices while maintaining them individually. This is different from the existing merge facility.
For example I might like to have aggregated statistics for lighting, or cooking, laundry, or heating (which might be comprised of a furnace and a few space heaters).
The same device might be in several orthogonal groups. For example, a computer in the kid’s room might be both in the electronics group and the kids group.
For the graphics people on your team, this would allow them to create interesting tree maps so that people can understand their utilization in different dimensions.