The crested top better resists crushing and deformation than a purely rounded shape. This is why you will also find a crest on most elbows (couters), knees (poleyns), and breastplates during this time period. Greaves and cuisses (thigh plates) also incorporated this crest…in shallow pieces, it stiffened the plate; on more exaggerated pieces (like the bascinet) it also formed a prow that made it harder to land a blow. In later centuries they took this even further, which is where you’ll see heavily-fluted Gothic armor.
This is also a time period when the pointed (Gothic) arch was very much in vogue, both in architecture as well as armor design. It’s as much a stylistic choice as it is a combat-effectiveness one. In other words, they probably thought it looked cool. 