I prefer a powerbank with a flashlight, which is one of the (minor) reasons the last powerbank I bought wasn't an Anker.
Something mulitifunctional is always a good thing.
Why not just use a phone is a good question.
Maybe if the powerbank was waterproof it could be used when an phone can't.
A phone is just a light shining in all directions, the powerbank could have a more focused light that shines as a beam, making it more like a real flashlight.
Many powerbanks are quite big. I have a 26000 mAh version with a light. I never tested it all the way to zero but I think teh light lasts at least 20 hours.
No it isn't super bright. But it can be quite useful when the power is out for a long time.
It's just an extra you won't use on daily basis. Just like many people carry a swiss pocket knife with all sort of tools that are rarely used. But still it's a good thing to be prepared.