That's incorrect. We actually know quite a lot about what angels do
10 In the same way, I tell you, there is rejoicing in the presence of the angels of God over one sinner who repents.”
14 Are not all angels ministering spirits sent to serve those who will inherit salvation?
Psalms 91:11
11 For he will command his angels concerning you to guard you in all your ways;
6 On what were its footings set, or who laid its cornerstone—
7 while the morning stars sang together and all the angels shouted for joy?
8 Each of the four living creatures had six wings and was covered with eyes all around, even under its wings. Day and night they never stop saying: “ ‘Holy, holy, holy is the Lord God Almighty,’who was, and is, and is to come.”
"Then I saw another angel flying in midair, and he had the eternal gospel to proclaim to those who live on the earth—to every nation, tribe, language and people."
"See that you do not despise one of these little ones. For I tell you that their angels in heaven always see the face of my Father in heaven."
So we know angels serve God and praise him guard people.