Defoliation and dormancy begin early for quite a few species within desert and chaparral climates. California buckeye can defoliate during the driest summer weather, refoliate for autumn, and then defoliate again for winter. They do what they must to avoid desiccation within their arid climate. Many more species do what they must to survive through winter.
That is why so many plant species are deciduous while they are dormant through winter. They shed their foliage when it is more likely to be a liability than an asset. Like summer dormant plants, they respond to inevitable and potentially detrimental weather. Moreover, they respond to seasonal changes of sunlight as well. Plant species are very perceptive.
With few exceptions, deciduous plants are nonconiferous or broadleaf species. More are endemic to regions to the north and south of tropical regions than within tropical regions. They know that sunlight is less intense and daylength is shorter while the sun is at a low angle during winter. Their defoliation coincides with the least usefulness for their foliage.
Most deciduous plant species are also aware of the sort of weather that they are likely to encounter during winter. Cold and stormy weather with wind and rain or perhaps snow is probably familiar to them. They know that foliage is not only vulnerable to damage, but is also burdensome to associated stems. In colder climates, it can accumulate heavy snow.
Foliage is the source of almost all wind resistance within foliar canopies that suspend it. Such wind resistance causes wind to dislodge limbs or blow vegetation over, particularly while soil is moist from rain. Defoliation eliminates much of such risk prior to the windiest and therefore riskiest storms of winter. Bare stems are more aerodynamic than foliage is.
Defoliation seems to happen at the best time, immediately prior to wintry weather. It even increases warming sunlight exposure during the darkest and coolest season of the year. However, defoliation is also messy while the weather is unpleasant for those who go out to rake it away. Without prompt raking, it clogs drainage of rain while it is most important.