Cloud seeding is usually attempted in order to increase the amount of precipitation falling. Given that there is a lot of supercooled water droplets in middle latitude clouds, and relatively few naturally occurring ice crystals in these clouds that will grow first by the ice crystal process then by accretion, we try to increase the precip by increasing the number of ice crystals. This is accomplished with a seeding agent, which is a material that acts as ice-forming nuclei.
A seeding agent must cause water to freeze into ice. Silver iodide has a crystalline structure that looks like ice, so it can help freeze water. (Dry ice was used in the first cloud seeding experiments, since it was readily available and it is cold enough to freeze small droplets of water without needing an aerosol particle nucleant.) The newly formed crystals first grow by the ice crystal process, then they accrete supercooled liquid water and grow into graupel, which may later melt into rain.An unfortunate outcome of many cloud seeding attempts is that too much seeding agent ("overseeding") is fed into the cloud(s).
Excessive numbers of ice crystals deplete the available supercooled water droplets, and the resulting ice particles are too small to fall out of the cloud and survive the trip to the ground without evaporating. It would appear that overseeding clouds causes them to dissipate.
We could use that to our advantage if we were trying to suppress the formation of heavy precipitation, such as hail. However, the thunderstorms that produce hail usually have so much available supercooled water, that overseeding attempts usually end up producing more hail than if the cloud was left alone!
Virga is usually seen falling from cirriform and middle level clouds. The region under a cloud, called the subcloud region is subsaturated (otherwise, there'd be cloud there!).
As precipitation falls through this space, it will evaporate. The larger raindrops/snow particles will not completely evaporate, so they reach the ground as precipitation.
From the middle or high cloud levels, however, the fall distance in subsaturated air is so large that practically nothing survives the trip.