Need for ICMP

Remember that IP is only responsible for getting datagrams from one host to
another, one network at a time. Each IP datagram gets sent as an individual entity
capable of following whatever path is available to it. Datagrams are moved across
whatever hosts, routers, and networks are capable of getting that specific chunk of
data closer to its destination.
In this model, any IP datagram can fail to get delivered for any number of reasons.
Some datagrams will get discarded simply because the next-hop router is
unavailable, and the current router is unable to forward them. Sometimes a datagram
will be destroyed due to the user on the sending system providing a nonexistent
destination IP address or port number to their local application. In all of
these cases, the system that detects an error will simply destroy the IP datagram
that’s failing, and move on to the next datagram waiting to be processed.