The Soviet army depends on the Tesla Reactor to run its base operations. Like Power Plants, Tesla Reactors are extremely important, but are very vulnerable. These structures must be protected at all times.
Spies are a particular nuisance.
While a few of your buildings, like your Construction Yard, provide their own power, most do not. To get them to operate at peak efficiency, you must provide them with power. That's the function of this structure. Power Plants put out a good deal of energy, but larger bases will require multiple Power Plants to operate effectively. Power Plants are physically weak and critically important, so keep them well guarded.