Bunkie board adds extra support to the mattress. The mattress feels more level. I recommend this for keeping the mattress bottom below the side panels of the bed, to have a recessed look.  This is helpful with hiding where the fitted sheet is a bit short on  a thick mattresses or if you don’t want the mattress to sit too high. I like that it came in two pieces as I am a small person and it was easy to carrymove.