To evaporate is to fade away or to literally turn into vapor. If you leave a glass of water out and the water slowly disappears, it's not being consumed by elves; it's evaporating.
People often use the word evaporate metaphorically because it produces a strong image of something disappearing like vapor. When the economy goes through a rough patch, funding for projects like education and the arts tends to evaporate or disappear. Your feelings for a mate might evaporate after you spend too much time with him or her.