Summary routes reduce the size of the routing table, thereby saving precious CPU and Memory resources on a router.
We also know that summary routes can limit the number of EIGRP queries on the network and improve convergence.
Here is the reason: Routers that receive an EIGRP query but already have a summary route in the routing table for that prefix will not transition to the Active state and propagate queries. Instead, they will immediately respond with a destination unreachable message, as illustrated in the following diagram.