For some people, they like having a different group name. I don't know why people like having different default group names, but they just do for some reason. I know that it is possible to use rules to give them another group on login, but some people don't want to go to that extent to give them that group.

https://github.com/lucko/LuckPerms/blob/master/bukkit/src/main/resources/config.yml#L387...L463

You can use /lp group <group> setdisplayname

https://github.com/lucko/LuckPerms/wiki/Command-Usage:-Group#lp-group-group-setdisplayname