Objection 1. It would seem that an indulgence does not avail him who grants it. For the granting of an indulgence belongs to jurisdiction. Now no one can exercise jurisdiction on himself. thus no one can excommunicate himself. Therefore no one can participate in an indulgence granted by himself.
Objection 2. Further, if this were possible, he who grants an indulgence might gain the remission of the punishment of all his sins for some small deed, so that he would sin with impunity, which seems senseless.
Objection 3. Further, to grant indulgences and to excommunicate belong to the same power. Now a man cannot excommunicate himself. Therefore he cannot share in the indulgence of which he is the grantor.
On the contrary, He would be worse off than others if he could not make use of the Church's treasury which he dispenses to others.
I answer that, An indulgence should be given for some reason, in order for anyone to be enticed by the indulgence to perform some action that conduces to the good of the Church and to the honor of God. Now the prelate to whom is committed the care of the Church's good and of the furthering of God's honor, does not need to entice himself thereto. Therefore he cannot grant an indulgence to himself alone; but he can avail himself of an indulgence that he grants for others, since it is based on a cause for granting it to them.
Reply to Objection 1. A man cannot exercise an act of jurisdiction on himself, but a prelate can avail himself of those things which are granted to others by the authority of his jurisdiction, both in temporal and in spiritual matters: thus also a priest gives himself the Eucharist which he gives to others. And so a bishop too can apply to himself the suffrages of the Church which he dispenses to others, the immediate effect of which suffrages, and not of his jurisdiction, is the remission of punishment by means of indulgences. The Reply to the Second Objection is clear from what had been said.
Reply to Objection 3. Excommunication is pronounced by way of sentence, which no man can pronounce on himself, for the reason that in the tribunal of justice the same man cannot be both judge and accused. On the other hand an indulgence is not given under the form of a sentence, but by way of dispensation, which a man can apply to himself.
Source: Summa Theologica (New Advent)