A bunkie board is similar to a foundation in that its main purpose is to add support and prolong the life of your mattress. The difference between the two is mainly height.
A bunkie board is ideal for not only bunk beds (for which it was first invented), but also for all other bed setups that need added support without adding too much height. Use a bunkie board over a slatted platform bed frame, and older box spring, loft beds, day beds, and even trundle beds.
Note that if you already have a platform bed with a solid surface or if you use an adjustable base, you do not need a bunkie board or foundation as those bed setups already give adequate support to your mattress.