The vampires in my setting have varying powers and weaknesses based on the magical grade of their blood. While the two higher-grade vampire types can survive in sunlight with no issue beyond losing access to some of their powers, for the two lowest grades, the celestial energy of the sun (not UV light, specifically the energy generated by the sun) disrupts their magical "biology" so much that they will quickly suffocate when exposed to sunlight.
But the weakest of the four grades has an even bigger problem. Not only does the sun's celestial energy kill them when directly exposed to it, but as long as the sun is even in the sky, even while they're indoors, they are rendered completely unconscious until the end of civil twilight next sundown.
I quickly realized this would be a good way to justify the "sleeping in a coffin" part of the vampire lore. Being completely helpless and almost indistinguishable from a fresh corpse as long as the sun is in the sky, they'd probably want the place they sleep to have some extra safeguard against exposure to direct sunlight in case something unexpected happens during the daytime while they sleep. So right before sunrise, they'd lock themselves in a sealed container. A coffin could easily suit that purpose.
But here's the issue: why would they pick coffins specifically? Why did it become tradition for the weakest vampires to sleep in coffins as opposed to, say, chests, or other similar containers that they could fit inside? What advantage did they have over the other options that made them so ideal for vampires to use as a bed?