When the principal of a school makes special mention of the high achievement of one of the students in assembly, it is not because the Principal doesn’t know, but because he/she wants to honour the high achiever by mentioning their achievements before all the other students and faculty. Almighty Allah knows everything but even then He asks the angels about His pious slaves so that the piety of the believers and their merit and distinction become evident to all.
The angels for the night come at the time of Asr when the angels from the morning are present. The angels of the two shifts assemble at this time and exchange duties. The angels of the night shift then leave their duty in the morning at Fajr, and the angels of the morning shift resume their duty. Thus, the two groups assemble again at this time.
Angels are very punctual. They know no such thing as procrastination or delay. Thus, when they are exchanging duties only the people who are punctual in their Salat are engaged in Fajr or Asr prayers. Therefore, we should try very hard to be punctual in praying the Asr and Fajr Salat so that when Allah (subhana wa ta’ala) asks the angels about us He is told that the angels left us while we were remembering Him and praying and not while we were sleeping, busy in games, watching T.V., or some other worthless activity.