Nebulous
Legend
Right. All you need are some monsters who are the type to try to surprise the PCs and suddenly a torch isn't looking so bad. Same deal with traps, which I think are hugely underused in many games. As well, secret doors provide places to rest, hide caches of treasure, shortcuts around dangerous areas, or pathways to new opportunities. If someone is traveling the dungeon looking for secret doors but is relying on darkvision, they might just be missing a lot of great stuff.
Yep, all of this. I think DMs (and myself included earlier in 5e) totally ignored this and treated darkvision as see everything all the time, when in fact, not having bright light is a problem sometimes.