The bicuspid and tricuspid valves are anatomical features of the heart, and their primary purpose is to facilitate the one-way motion of blood through the heart as it contracts and relaxes. Essentially, they are one-way doors that prevent the blood from flowing backwards.
Because the heart is a muscle, it can only contract and relax. When it contracts, it decreases the volume of its internal chambers, thereby increasing pressure on their contents and causing them to flow out of any available opening. Each chamber must have an entry and exit point in order to be of any use to the circulatory system, but both points would, by default, be equally viable places for the high-pressure fluid to flow through. This would negate the purpose of the heart's contractions because it would fail to produce any significant directional flow. Therefore, the heart needs a way of blocking flow that would go in the wrong direction, and this is the function that the valves perform; they open for fluid flowing from the atria to the ventricles, but cannot open to permit the reverse.