On recent versions of Windows, neither the telnet client nor the telnet service is installed by default, but they are still available as features.
Years ago, they were used as a terminal service system to manage remote hosts.
Currently, a lot of people are still using the telnet client to test if a TCP port is open.
But starting with Windows Server 2012 R2 and Windows 8.1 you don’t need Telnet anymore to achieve this because there is now the
Test-NetConnection cmdlet sends an ICMP Echo request (PING) and in addition, if you add the -port parameter, it tests if the TCP port is open.