Login command is used in VTY for password that is specified to be checked at login. If you do not use login command you will not able to use the specified password for the vty to login.
Login can also be used with login local. In which you can config a username and password on the router to be auth.
below is the link.
Hope this helps.
Login is used to control VTY access, it generally has 4 cases when AAA is not configured, and with AAA it serves another purpose:
One of the following methods must be explicitly configure otherwise there can't be VTY access to the router.
1. "no login" > No password is required for VTY access
2. "login" + "password xxxx" > This password is required for login.
3. "login local" + "username xxxx password xxxx" (global configuration) > a local configured username and password are required for login.
4. "login tacacs" + "tacacs-server host x.x.x.x" (global configuration) > Use TACACS or Extended TACACS server for login.
With AAA it can be used to specify a custom AAA authentication method using the "login authentication xxxx" command under the VTYs.
NOTE TACACS+ can be enabled only through AAA commands.
I hope that i've been informative.