Why did explorers need to claim the land they found?

3 main things- GOLD, GLORY, and GOD.

GOLD- the explorers wanted to get Natural Resources like gold and silvers for both themselves and their countries.

GLORY- the explorers wanted to be famous, so they claimed all land they found. this resulted in the glorification of the explorer and their country.

GOD- the explorers believed that it was their right and duty to conquer other land and spread Christianity. They thought God had meant for them to do this.

Hope this helped! =D