John,

Our primary motive for any action should be "love of God". That's not "merit making" it's an act of love, a proper 'response' to the love of God.
This practice is actually a marvelous way to "unify" the various "parts of life" so that we don't seem to have 5 or 6 lives! (husband, father, professional, church goer, political, intellectual life, etc.). No: one life, all things done out of an intentional love of God.
Very serenity providing.
So, rather than "making merit" and then being disappointed (losing serenity or charity when things don't go right) if one's motive is the love of God in some action and it goes badly, it's far easier to deal with because we can honestly say to God..."that's Your issue, not mine, Papa, I was loving You with my all".
Rectifying the will is a very old practice going back to the earliest times of the study of the interior life, probably the desert fathers or somewhere like that.
Not an Opus Dei invention, but it's certainly taught by OD.