Except for those which the Church says is applicable only to the poor souls in purgatory, such as the visit to the cemetery from Nov. 1 to Nov. 8, plenary indulgences gained by the faithful are gained for themselves. Of course they may also be offered to the poor souls by way of suffrage, but we gain them primarily for ourselves. But what is the point of this if, as you say, it is better to just offer all indulgences we earn to the poor souls? Why doesn’t the Church just make a decree that all indulgences earned here below should be applicable only to the poor souls, and not for ourselves?
I think the reason is this. Indulgences remove a part or all of the temporal punishments due to our sins. I will now use an analogy that I know is not perfect, but will hopefully explain my point. Let me just say that I happen to break ten windows of my neighbor while I am living. (Breaking the windows represent my sins.) Although I apologized and was forgiven, I am still under obligation to fix those broken windows, right? (That represents the temporal punishments due to the moral damages caused by my sins.) Now, if I can somehow repair those windows while I am still living by earning indulgences, then I would rather do that than offer all my indulgences to repair other people’s damages while I leave my own broken windows unfixed. If you want to do that, it’s OK. But that does not sit well with me. In fact, I do the same thing with respect to my earnings. I don’t donate all my earnings to the poor, and leave the care of myself to Divine Providence. I’d rather donate some of my earnings, but save some for myself as well. And that’s how I look at indulgences. I want to use them to make reparation for the moral damages caused by my own sins. I am willing to donate some, but also leave some for myself. I guess this is the reason why the Church teaches that indulgences that are granted to us, not exclusively for the poor souls, are for our use, but may be applied to the poor souls by way of suffrage.
Note: I am not asking everyone to do it my way. It is just a purely personal preference. And sure, I will continue to earn and apply all my plenary indulgences earned every day of the month for the poor souls, except for one day in the month which is reserved for me. I will still have many friends in Heaven in the end.