Why IP datagram be encapsulated

Question

Why an IP datagram be encapsulated in the other IP datagram?

Answer 1

An IP datagram be encapsulated (taken out as payload) within an IP datagram, termed as Encapsulation to act as  means to modify the normal IP routing for datagrams, by delivering them to an intermediate destination which would or else not be selected based on the (network portion of the) IP Destination Address field in the original IP header. Once encapsulated datagram arrives at this intermediate destination node, this is decapsulated, yielding the original IP datagram that is then delivered to the destination pointed by the original Destination Address field.  This utilization of encapsulation and decapsulation of a datagram is often termed to as "tunneling" the datagram.