Think of the confusion we would have if every bishop followed his own inclinations. The Church would literally fall apart in a very short time. The stake president serves as an adviser to the bishops. Every bishop knows that when he has to deal with a difficult problem there is one readily available to whom he may go to share his burden and receive counsel. He provides a secondary measure of safety in determining those worthy to go to the house of the Lord.
Bishops are very close to their people.
They live with them as neighbors. Sometimes they do not have the heart to refuse to grant a recommend even though the patron's worthiness may be somewhat in question. But the stake president also interviews. Until Wilford Woodruff's time, the President of the Church signed all temple recommends. But the burden became too heavy, and stake presidents were given the responsibility.