Anti-Transpirants help keep broadleaf evergreens from drying out in winter. Broadleaf evergreen trees and shrubs like rhododendrons, boxwoods, mountain laurels, hollies, andromeda, leucothoe, cotoneaster, euonymus, and inkberries are all subject to the effects of winter drying, otherwise known as winter-kill. A simple dynamic is responsible for this damage: Leaves are always transpiring, even in winter, and as they transpire they give off water. When it's very cold, roots lose some, if not most, of their ability to absorb water. The result is a water deficit for the leaves, which then can wilt and turn brown. Wind can further exacerbate the problem.
...the most effective protection against winter kill is snowfall.
If snow is deep enough to cover leaves, it enshrouds the leaves in a moisture laden environment and protects the leaves from drying winter winds. The second best protection against winter-kill is to wrap broadleaf evergreens in burlap.
Annual snowfall amounts are not reliable, and even in the deepest of snowfalls many larger broadleaf evergreens are still exposed, it's best not to rely completely on mother nature for protection. Burlap is an expensive and also unsightly option. A third alternative is to apply a winter anti-transpirant to broadleaf evergreen leaves. These materials slow down transpiration rates thereby greatly reducing water deficits in leaves, winter drying, and winter-kill.