The Council of Trent says:
CANON XI. If any one saith, that men are justified, either by the sole imputation of the justice of Christ, or by the sole remission of sins, to the exclusion of the grace and the charity which is poured forth in their hearts by the Holy Ghost, and is inherent in them; or even that the grace, whereby we are justified, is only the favour of God; let him be anathema.
Canons on justification
As we can see there were theological opinions stating that people are justified by simple "imputation of the justice of Christ". It would mean, that this is just a legal act of making Christ justice to belong to someone else too. As we know from the Council of Trent - that's not the case and it is a heresy.
What is bigger? What brings more glory to God - just a legal act or truly transforming a man by grace? Isn't such a "transformation" of a man more glorious, more showing the omnipotence of God? God can not only legally impute Christ's justice, but through His grace He makes us just.
Therefore more glory is in making us just than in simply imputing Christ's justice to us. It's obvious therefore, that for greater God's glory He gives us grace to make us just, so we can glorify Him forever in Heaven.