What was the reason for the Japanese attack on port moresby?

There is a port moresby on the northwest coast of Coral Sea. It is Australia's air and naval base in Papua Peninsula. The Japanese army believed that the occupation of this port could protect rabaul and its military sites in New Guinea, and also provide flank protection for the next attack on New Caledonia, Fiji Islands and Samoa. So after the Japanese army occupied rabaul in June 1942+ 10. Its base camp issued an order to the joint fleet: seize port moresby as soon as possible.