They offer shade and camouflage and soften a fence line. When leafless, they allow a fence to dry out instead of trapping moisture against the wood with a year-round mantle of thick growth.
Annual vines are, in a manner of speaking, deciduous.
They cover in a single season if you take care to prepare the soil, weed, and help them to reach the fence as soon as they show the inclination to climb. The seeds are large and easy to plant as soon as the soil can be worked and is frost-free. Wait until the ground is warm to plant morning glories and don't fertilize the soil. And they need little care aside from occasional pruning. If they bloom in the spring, prune after blooms have faded because some bloom on year-old wood.