The decision to support Columbus' voyage was multi-faceted. The Reconquesta and increasing conflict against the Ottomans led Isabella and Ferdinand to look for allies, with the ultimate goal of bringing the fight to Jerusalem. So one aspect is that they hope the voyage would help them find such allies. Another is that it would open a new route to Jerusalem in addition to the Mediterranean.

Of course, there is no small consideration of possible profits. The Reconquesta was an expensive endeavor.

Finally, there is also competition against the Portuguese, who had recently made discoveries in the east Indies. Both Isabella and Ferdinand saw Portuguese as political and economic competitors, even if one that may be allies against the Moors.