It’s different for different games, but generally speaking, the point of NG+ is to reward players for beating the game and give them a new way to experience it. And again, the stuff that carries over between play-throughs and the stuff that changes varies depending on the game. In Starfield, the only thing you keep are your level & the skills you’ve unlocked. It seems like they added it for people who like to make a bunch of new characters to experience quest lines differently. So instead of starting from scratch, you can go into NG+ on your current character to replay quests & you have a more tangible reward from your last run in the form of XP. And other stuff, but no spoilers.