John Calvin on Mark 8:22-23

This miracle, which is omitted by the other two Evangelists, appears to have been related by Mark chiefly on account of this circumstance, that Christ restored sight to the blind man, not in an instant, as he was generally accustomed to do, but in a gradual manner. He did so most probably for the purpose of proving, in the case of this man, that he had full liberty as to his method of proceeding, and was not restricted to a fixed rule, so as not to resort to a variety of methods in exercising his power. On this account, he does not all at once enlighten the eyes of the blind man, and fit them for performing their office, but communicates to them at first a dark and confused perception, and afterwards, by laying on his hands a second time, enables them to see perfectly. And so the grace of Christ, which had formerly been poured out suddenly on others, flowed by drops, as it were, on this man.