Why do we need sag in transmission lines?

Sag is mandatory in transmission line conductor suspension.

This is because it protects the conductor from excessive tension.

In order to permit a safe level of tension in the conductor, conductors are not fully stretched; rather they are allowed to have sagged.

If the conductor is stretched fully during installation, wind exerts pressure on the conductor, hence the conductor gets a chance to be broken or detached from its end support.

Thus sag is allowed to have during conductor suspension.