It also means you have great reference material for your custom monsters!
Monsters are just more disposable, they are game pieces. They exist to combat your PC so streamlining them makes them easier to run and take a load off your mind. You don't track a monster's arrow because they'll be dead or captured in 5 minutes.
NPCs are PEOPLE. They have more layer, more depth, you can have them adapt to situations much more easily. A NPC could fill more than one role in combat and would have their own stuff outside of combat. They're the casters with out of combat spell, the bandits with gear outside their weapon, or the nobles with connections. You keep track of how many Alchemist's Fire an NPC has used because they might retreat and come back later in the day.
NPCs are crafted, Monsters are store bought.