Your talking about a morningstar, which is actually a weapon type separate from the mace. You can use fantasy author Ciarra Ballantine’s handy chart for distinguishing your bludgeoning weapons if you like, it may help you when it comes to telling the mace, flail, and morningstar apart.
As for why the spikes? All the better to bludgeon you with, my dear.
The primary use for these weapons in combat was against knights and other armored foes (though some priest orders in the Catholic church were famous for carrying them and wielding them against unarmored peasants, this is where the Cleric in DnD comes from). The basic idea is you crack the plate like a tin can, the point of a mace is to just drive force through the armor until it hits the person inside, or crushes them. The spikes add to the benefits you get off the mace. They’re for punching through the armor and into the body.
A sword can’t beat someone through their armor, it’ll damage the edge. You can pierce the armor by holding the blade of the sword in your hands and driving it in, which is the point of the estoc. Or you take the hilt and start beating on their armor with that.
A mace cuts out that middleman, allowing you to hammer someone to death and force your way through their armor. The morningstar adds spikes to that equation. So, if the mace is a hammer then the morningstar is you wanting to put a nail into someone so you’ve soldered the nail onto it to put the nails into them.
If it helps, think about it like going after someone with a baseball bat. Baseball bat is good, nails in the baseball bat is better.
The flail is the same way. You put a spiked ball on a chain and swing it about to hit people with it to bust up their armor, because you get more force from a steel ball spinning on a chain than you generate with your arm.
As for the benefits? It’s really a question of how badly do you want to fuck that other guy up, and how viciously do you want to go about it.
You can go into combat wielding a flail in one hand and a shield in the other against a knight with their sword and gain a significant advantage because it moves in directions that are difficult to counter. It generates enough force to damage both the plate and person inside.
I mean, there are other reasons why someone might choose to carry these weapons but those are some of the big ones. As a function of its design, the morningstar has a greater reach than the mace.
It’s a club, it doesn’t take that much finesse to wield in comparison to a sword and its highly effective at what it is designed to do. The bludgeoning weapons are fairly uncomplicated, though they make a big mess.
From a writing standpoint in your fiction, the maces and morningstars tend to carry some stigma in comparison to swords. They’re in the same family as the club. Basically, they’re treated as thuggish weapons. As opposed to the blade, which is a noble, elegant weapon based in skill and finesse. Its treated as less legitimate. Or, it belongs to the Crusaders and used by religious groups only.
Take these preconceptions into account when using the mace, but don’t hitch your cart to them. A character can be defined by their weapon choice, but don’t put too much stock in the conventional fictional definitions. A character who uses a mace is not necessarily a brute, just as a character who uses a sword is not necessarily a noble warrior or someone wielding a rapier having a rapier wit. Fiction and cultural motifs often have little to do with the reality of what the weapons were actually for, and end up getting it wrong more often than they’re right.
Axes also fall under another category. They’re the weapon of the savage, brutish, primal warrior, but they’re not. They’re just another weapon type.
It’s a choice, a combat approach.
The sword is the weapon of kings, a symbol of civilization and nobility. The weapon of the hero. A hero who wields a sword is more noble than one who wields another “lesser” weapon type. It’s not. It’s just another approach to combat, a tool.
Understanding these stereotypes is important because they will follow you and influence you, whether you realize it or not. They’ll also influence your reader. By understanding that they’re there, you can account for them and combat them in your narrative. In ignorance, you’re at their mercy. Countless other storytellers have already laid this groundwork for you, but you do get to decide what you’ll do with it.