Using their roar, an alpha can empower and stimulate their betas so they can accomplish high level tasks, heightening their strength, power and fortitude in dangerous situations such as being able to fight back against enemies. The alpha will also roar in to restrain potentially aggressive shapeshifters, or those who are rulled by their bloodlust by forcing them into submission or weakening them.
In 1x07 - Night School, Peter Hale was able to force Scott McCall - his werewolf progeny - to shift into his werewolf shape by roaring. He was then able to place Scott under a form of mind control. Deucalion also roared to force Scott to shapeshift in 3x12 - Lunar Ellipse. In 2x02 - Shape-Shifted, Derek Hale roared at his newly-turned beta Isaac Lahey who was under the sway of the full moon and Isaac became rational again.
After Scott became a True Alpha and gains a pack comprised of members of several different species (werewolf, coyote, human, banshee, kitsune), his roar has specifically been shown to work in the manner of a clarion call as well as for the usual effects.
Scott was able to force Malia Tate to shift from her full coyote form to her human guise with his roar in 3x14 - More Bad than Good. Simultaneously, his voice affected and stimulated Isaac allowing the latter to break free of a bear trap. His roar was also clearly heard by his packmates Stiles Stilinski and Lydia Martin despite both of them being miles away from Scott's location at the time and them simply being a human and banshee respectively. For weeks afterwards, Malia was unable to trrigger her shifting afterward until Scott stepped up to mentor her.
In 5x15 - Amplification, Scott let out a roar while he and his beta Liam Dunbar were being electrocuted by Eichen House security guards. His roar boosted both their preternatural strength and endurance, and both werewolves overcame the electricity and single-handedly knocked the guards unconscious. His roar also affected his pack members: Malia responded with her werecoyote eyes glowing brighter and he was clearly heard by Stiles and Kira Yukimura.