Potion of Night Vision removes darkness rather than lighting it. While the effect lasts every block renders as if it were brightly lit, with no torches placed and no change to where mobs can appear — a cave you can suddenly see across is still dark enough to spawn hostiles behind you.
Brewing starts with an awkward potion and a golden carrot. Redstone dust stretches the duration from 3 minutes to 8, while glowstone does nothing, because the effect has no stronger tier. Gunpowder turns it into a splash potion of night vision for handing the effect to someone else, and a fermented spider eye converts it into a potion of invisibility, so a shelf of these is really a shelf of two effects.
Underwater is where it earns its place. It brightens the sea floor, so an ocean monument, a ravine or a flooded cave reads at a glance. On land it lets you mine and explore without lighting every corridor, which matters when you want a cave left dark for a spawner or a mob farm.
Two things are worth knowing before you rely on it. The screen begins to flicker in the final seconds, which is the cue to drink again. And the Darkness effect around a warden or inside an ancient city overrides it — night vision will not stop the pulsing blackout from closing in.