What is the use of "export" command?

export makes a variable something that will be included in child process environments. It does not affect other already existing environments. In general there isn't a way to set a variable in one terminal and have it automatically appear in another terminal, the environment is established for each process on its own.

Adding it to your .profile makes it so that your environment will be setup to include that new variable each time you log in though. So it's not being exported from one shell to another, but instead is instructing a new shell to include it when it sets up the initial environment.