One reason that lions roar is because they want to communicate with other lions. Most lions choose to roar at night because the sound travels further when the air is thinner. One of the reasons that lions roar is to warn other lions to stay away from their territory.
Lions use their roar as a way to remind other lions that they are in control over an area. Another reason for the lion's roar is to reconnect with lions who might have traveled farther away while hunting or roaming. This allows the lions to find one another again and to check in with one another. All lions have a unique roar, which helps them to understand who is calling out.