They are the angels that are writing the mankind’s deeds. Allah (swt) says:

{إِذْ يَتَلَقَّى الْمُتَلَقِّيَانِ عَنِ الْيَمِينِ وَعَنِ الشِّمَالِ قَعِيدٌ % مَّا يَلْفِظُ مِن قَوْلٍ إِلَّا لَدَيْهِ رَقِيبٌ عَتِيدٌ  }

(When the two receivers receive, seated on the right and on the left. %  Man does not utter any word except that with him is an observer prepared [to record].) (Qaaf: 17-18)

People in documenting matters only trust what is registered and recorded which secure the safety of their dealings, such as the contracts and others, so where will be no place for doubt, heedlessness or forgetfulness.

Allah (swt) says to them: I will notarize your actions the same way you are familiar with in your life matters. So, the issue of recording and writing down the deeds is not useless; it has a purpose. Mankind’s doings are registered for him precisely, punctually and honestly, which Allah (swt) has guaranteed for us by those who are in charge of it; the angels.