They may not lose their bright green foliage, but they do slow down and rest. The sap flows more slowly. No fresh growth is produced. Perhaps one could say they are dozing. In tropical regions, plants seem never to rest, but they do.
All plants have dormant stages.
Just like humans, they need time to regroup and recuperate from a long season of producing buds, fruit, new leaves, roots and all that goes with being productive plants. People need sleep. If they do not get enough sleep, they suffer from sleep deprivation, become irritable, confused, unable to concentrate. Plants also need their rest. If deprived of their dormancy, they become weak, will not bloom and eventually may succumb to disease or insect infestation.