Hope this helps:
"If you do not use the optional wildcard mask, the EIGRP process assumes that all directly connected networks that are part of the major network will participate in the EIGRP routing process, and EIGRP will attempt to establish EIGRP neighbor relationships from each interface that is part of the overall Class A, B, or C network."
If you define an IP address that belongs to Class C network, the EIGRP process will assume the mask 255.255.255.0 and its corresponding wildcard mask. However, the recommendation is configuring the wildcard mask. For example, if you have configured multiples interfaces with subnets that belong to the same Classful network, but we don't want to some interfaces don't participate in the EIGRP process, in this case, the wildcard mask must be used for matching the specific interfaces.