Sorry, should've been more clear, but I'm currently running a more "spy"centric campaign (think Dishonored, if you've played it). So a lot of the characters are recurring and have a bigger impact on the narrative. I use it for them. But like NPCs, or a PCs father, etc. I don't bother lol.
Don't get me wrong, I still use monsters and stuff from the monsters manual, but to give some fun challenges, I figured "instead of throwing 20 smaller enemies at my players, why not roll up 2 characters as the "generals" of this army, and that will be their combat for this session".
I thought Itd be fun for my players to fight someone who had as insane abilities that they do. After all, they're not the only adventurers or power seekers in this world.
I've tried my best (this is my first experience DMing a campaign) to recreate the feeling that Oda does in One Piece. The world is alive and vibrant. Sure, we are following our players, but that doesn't mean the world revolves around them. Stuff is going on without them there, so they might show up in the middle of a giant thunderstorm, or a raider attack, or nothing could happen at all, until they leave.
My players have liked it so far, telling me it gives them plenty of story hooks that they could follow if they wanted to.