Why do we give so much importance to St.paul (a self-appointed Apostle)and his letters?

St Paul did not appoint himself as an apostle but Jesus Christ appointed him.
Originally there were twelve Apostles chosen during Christ's lifetime;but
apostleship was extended later.St Paul was the most famous addition called by
Christ,like the Twelve,to be an ambassador for Christ(2 Cor.5:20).Paul wrote
to the Galatians:"From Paul,an apostle appointed not by human beings nor through
any human being but by Jesus Christ and God the Father who raised him from the
dead" (Gal 1:1).St.Paul's letters are exemplary not only for their style but also for the
variety and importance of their doctrinal teaching.