It's to control the amount of power the govermint is going to allow you to consume. Damage the infrastructure, force people to do without no matter what the consequences (chronically ill using life support devices at home, heat for the elderly, etc.); just a taste of what's to come.

I think some of the people sabotaging power grids are idiots and fail to recognize how they are damaging their very own infrastructure -- So they shoot up a transformer or two, but then get home and find they have no electricity for a few weeks.

OTOH, some who do this are terrorists, domestic most likely, who are testing to see how much damage they can do to the local infrastructure.

Let's hope they don't figure out how to really bring down a wide-area power grid, which is easy to do.