Both male and female lions roar, but the male has a roar that can travel up to 5 miles. At least, that's how far away a human can hear it. it's unclear how far another lion can be to still hear the sound, but almost certainly lions can communicate with each other for well beyond that distance. Only lions that are part of a pride roar. Solitary lions stay relatively quiet. That may give us a clue.
Lions are very social animals, and they frequently need to communicate with each other. A nice loud roar can serve that purpose rather well. But what would they communicate with each other? The sound of a lion's roar can be like a quick phone call home.
A male lion may roar to let the other members of the pride know that all is well. The female lion's roar is usually different than the male's roar. They are much quieter than the large male lions. A female can call out for roaming or lost cubs, and she can give a roar to bring the other adult pride memebers back to help in case of danger.
So a lion's roar can be explained rather simplistically. It is used to communicate with the rest of the pride. It's like a quick status update or a call to come home.