In one of my early assignments we had a little shrine to St. Monica in the public chapel. It was enormously popular. One doesn't have to spend much time in ministry to realize that a large proportion of the prayers of this world come out of the anxiety of mothers for their sons in trouble. St. Monica is the perfect intercessor for these intentions.
Another area for which I have recommended Monica as an intercessor is in the area of sexual purity. St. Augustine had his troubles in this area, as he freely admits, and chastity was a stumbling for him in his discernment of Catholic Christianity. Therefore, if we take seriously what we pray in the collect today, that God received Monica's tears for the conversion of her son (the current English translation about God being moved misses the mark a little bit) we must conclude that her prayers helped Augustine accept the grace of chastity.
St. Monica, pray for us.