Watcher Angels
The purpose of the Watchers were, obviously, orginally to watch over mankind. Similar to Guardian Angels. Unfortunatly, they fell lustful for the women of earth. The Watcher Angels, also known as Girgori, are a group of angels who betrayed God and came to earth to have sex with the women of earth. They had children who became giants 300 cubits tall known as Nephilim. Because of their sin, God flooded the earth and made earth sin-free killing everyone and thing except Noah and his family. There are 200 Watchers who betrayed God. The Watcher angels are taller than giants and remain silent. The Watchers who taught humans the way and knowledge of angels (astronamy, writing, painting, signs, sorcery, etc.) were chained in the depths of earth until the time of Judgement when they are released and judged (Seals of Judgement spoken of in Revelations). The leader of the Watchers is the archangel Semyaza. (All this information is from the Book of Enoch, though Watcher Angels are talked about in other books of the Holy Bible)
When men began to multiply on earth and daughters were born to them, the sons of heaven saw how beautiful the daughters of man were, and so they took for their wives as many of them as they chose. Then the Lord said: "My spirit shall not remain in man forever, since he is but flesh. His days shall comprise one hundred and twenty years." At that time the Nephilim appeared on earth, after the sons of heaven had intercourse with the daughters of man, who bore them sons. They were the heroes of old, the men of renown.  - Genesis 6:1-4 Stupid Watcher Angels