There are a few examples in the New Testament wherein the choosing of people especially in leadership positions required sometimes extensive times of prayer. Jesus prayed a whole night for His twelve apostles. These very apostles had to replace a deceased member of their group (Judas Iscariot) and prayed for a sign.
There is nothing a disciple should do without asking God first and awaiting His strict and perfect instructions. And even if the Early Church may have seemingly chosen people without God’s explicit direction at one point, prayer afterwards can also mean, we await His explicit consent.