Ah, this is a difficult concept for most of us. We tend to think of mercy and justice as opposing concepts. However, Catholics believe that in God both act constantly, perfectly, and without conflict. The real problem with merely following the Law is that it isn’t sufficient to make up what we lack. The question must be asked, “Since man, through Adam, has thrown the gift of Original Grace back in the face of God, what can man do to deserve another chance to get that grace back?” The answer is, nothing because nothing we can do can ever be enough to deserve such a gift. Now in His mercy, God desires to give us this gift. But God cannot just set aside His justice for it is part of His very nature - along with mercy and love. In fact, theologically speaking, God is the ultimate definition of justice, mercy and love. It would be an offense against justice for God to give man what we do not deserve; especially when it was willfully rejected and especially when we continue to sin. Now, I’m not talking vengence here, it’s about what is just. Man had to make up for the offense man had caused. But due to the alteration of our very nature caused by Adam, no man was capable of doing so. No man’s motives could be pure enough or effective enough to accomplish it even for himself let alone all of humanity. Observance of the Law is a good thing and we Catholics are required to observe our religious laws as well. But mere observance does not deserve heaven. The sacrifices of the Mosaic Law were good and effected a level of purification, but, again, they were never sufficient to warrant Heaven. However, God was capable.
In justice, man needed to make up for the offense but only God was capable of doing so, therefore in His mercy He became man. Could He have chosen to accomplish this by another means. I have no doubt for God is all powerful. But I’ll take a lesson from Job. I was not around when God set the stars, planets, and moons on their course. I cannot, by my word, cause the sun to rise or set. I was not involved in God’s decision of how He would bring about our salvation after Adam’s Fall so I will simply accept that He did it they way He wanted and that His way is best even though I am completely incapable of fully understanding it.
Now, honestly, that is about as far as I can go on this particular topic. I know that there are depths to this concept that go far beyond my capabilities so if anyone cares to jump in or has a good recommendation for materials on the topic, please do!