The purpose of Theology (which is the study of God) is spiritual nourishment. For Christians, God nourishes us with truth, law, and sacrament. That is the food which Christ instructed Peter (the first Pope) to feed his flock with. Thus, to study theology is to seek the maximal amount of spiritual nourishment. And while it is not possible, at least as a practicing Catholic, to go completely unnourished, one can, by being completely or deliberately ignorant of theology, go somewhat undernourished.
For one thing, it is hard to love what you do not know, and if the purpose of life is to know and love God, then theology is quite important. Christ reminds the Devil of this when he says, ‘Man shall not live on bread alone, but on every word that comes from the mouth of God.’ Again, God’s word is our spiritual food.
Start with Theology for Beginners or Theology and Sanity. Then read through Brant Pitre’s Jesus and the Jewish Roots of the Eucharist.