Sockets do not always need to have an address. The socketpair(2) function, for example, creates two sockets that are connected to each other, but without addresses. They are, in essence, "nameless" sockets. Imagine a red telephone between the U.S. president's office and the Soviet Union, during the Cold War. There is no need for a telephone number at either end, because they are directly connected. In the same way, the sockets created by socketpair(2) are directly connected and have no need for addresses.
Sometimes in practice, one of the two sockets in a connection will have no address. For a remote socket to be contacted, it must have an address to identify it. However, the local socket that is "placing the call" can be anonymous. The connection that becomes established has one remote socket with an address and another socket without an address.
Sometimes you don't care what your local address is, but you need one to communicate. This is particularly true of programs that need to connect to a service, like a RDBMS database server. Their local address is only required for the duration of the communication. Allocating fixed addresses could be done, but this increases network administration work. Consequently, address generation is often used when it is available.