Historical texts like the bible, book of Enoch, Jasher, Giants, and countless accounts from all over the earth (from the begining to now). All create a pretty detailed answer to the original question.
-1/3 Angles fell after choosing to "get it on" with humans.
-Wanted to go back to heaven but were not allowed.
-So they gave hidden (occult) knowlege to men In exchange for worship n women
-had nephilim babies and tried to corrupt all humanities DNA so messiah couldnt be born
-flood happened cuz only 8 people were uncorrupted by fallen angel DNA
-all nephilim hybrids died and their souls are the "rephiem" (demons and evil spirits)
-now the 2/3 good angels are combating the 1/3 fallen.
-they also help to assist with Gods plan and are "messengers"
For those that have not witnessed supernatural evil. Or dont believe that the supernatural exists . This might be a hard concept to come to grips with. But it is real as u and I are. There is a vail that exists as a barrier to protect us so that evil doesnt run amuck. The angel help to maintain it until an unspecified time. They also protect those with faith from the fallen angels and demons that have free range with unbelievers.