The Power Plant mainly exists as a restriction to Transformer upgrades in the player's base. As transformer upgrades will enhance the boosts they provide, the Power Plant is indirectly crucial for base defense.
When destroyed, all Transformers' health will be reduced to an estimate of 10%, making them much easier to destroy. The damage scales on their current health, so they will not be destroyed entirely no matter how much damage was taken before. This also applies to the Heavy Transformers.
On rare cases, like in Campaign maps, a Power Plant can appear even though no transformer is on the map. This means that the transformer is merely another destructible miscellaneous building in this case.