Why did the portuguese was to establish strong hold in ports in India and Asia?

This was during European exploration and expansion. The whole reason Europeans were exploring was to find other ways to stimulate their economy after the Arabs cut them off from Asian markets. Vasco de Gama was the first explorer to go all the around the southern tip of Africa and back up to India. He traded his poor quality goods for highly valued cinnamon and pepper and returned to Portugal. He came back a few years later with a fleet of ships to establish fortified trading posts. This was all to reclaim the Asian markets from Arab dominance.