Direct addressability. Removes the need for slow, expensive intermediaries like NAT boxes and certain types of proxies by allowing security policies to be applied independently of network addressing.

Simplified network planning. Enough address space to make an address layout based on your actual needs rather than being forced to carve up your limited v4 allocation in weird ways. You no longer have to guess what the right long term size of your subnets are (they are always big enough) or worry about putting unrelated machines on the same subnet because you are out of space for new subnets.