Why is openssh-server needed to receive incoming ssh connections?

Solution 1:

The TCP/IP network stack in the Linux kernel only provides a means for data transport: you need a program that listens to the incoming data, processes that, and sends output data back. Such programs are usually called "servers" or "daemons"; that's why you need to run the "OpenSSH server" in order to receive SSH connections on your machine.

Solution 2:

The ssh connections to your computer are handled by an ssh server. If you do not have a server running then the connection request just gets ignored. Similarly: if you want to check a web page then the computer you are connecting to has to run an http server. In a default Ubuntu installation neither of these services are installed.

Solution 3:

for more information about SSH protocols read this page :

Open-Ssh Web Page and help ubuntu

Solution 4:

OpenSSH server is not needed, but as Riccardo explains you will need some SSH server to accept the ssh connection and do something useful with it. And if you don't have a good reason to use something else, using OpenSSH is a good idea (it's the best known & most used SSH server, and thus well-tested).