Hello Assaf,
packets with a destination interface null0 are silently discarded by the router.
So the router that generates the EIGRP summary route adds a routing entry for same prefix to null0.
If later the router receives packets destined for un unknown subnet within the summary route IP prefix it can discard it.
This avoids to send the packet to some other router with the risk to receive it back (for the EIGRP summary route) that would be a routing loop: a packet going out and back between two routers.
It avoids to send in the network packets that will be discarded when their TTL becomes 0.
Hope to help
Giuseppe