Why is it important to configure the bandwidth on an interface for EIGRP?

Answer 1

Simplified answer.

EIGRP is a router routing protocol.

Routing protocols inform other routers on a network about the current network state and available paths for traffic to flow.

To do this they use bandwidth. Everytime a state changes (i.e. an interface goes down), every router will try and tell all the other routers about the state change, setting off a chain reaction.

This chain reaction can quite easily flood the network, thus prevent the network from functioning. Therefore, setting the bandwidth on the routing interface provides a way of bottlenecking the state change information and preventing it from consuming all the available bandwidth and helping to maintiain service .

Hope this helps.