The Power Plant plays a unique function that has a bit of nuance associated with it. You will need to build a Power Plant in order to build Transformers, and you will need to upgrade your Power Plant in order to be able to upgrade Transformers. What’s more, the Power Plant is tied to the overall health of your Transformers. This means that if the Power Plant is destroyed during a base attack, all of your transformers will suffer a significant loss in health, making them easy targets.
So why are your Transformers so important? They provide an attack speed boost to all defense platforms within range.
A single defense platform will receive the full attack speed bonus from the Transformer. Turrets that benefit the most are the ones with slow reload speed like the Howitzer and Railgun.
A single Transformer can power multiple defense platforms. However, the maximum attack speed bonus will be evenly split between them. For example, this Railgun by itself receives the full 70% attack speed bonus.
But when another turret is placed within range, they now each gain a 27% bonus. This split would continue for each additional defense platform in range.