To allow full nodes to operate with limited bandwidth, i.e. to allow decentralization.
DAG makes it much easier to handle intensive data flow because of multicast. A single transaction easily fits into MTU and if it's not received by few of the endpoints it's not a problem. Even more, TPS bursts are automatically smoothed, a full node needs majority of the transactions only if it sends or receives money, in all other cases it needs only to verify PoW and rebroadcast transactions to its neighbors (if multicasting is disabled).