If your game has critical strikes, what does it mean to you when either a player or enemy critically strikes? Was it the display of excellent combat skill that landed a precise blow? Was it a powerful swing that was able to hit really hard? Was it a "lucky" shot?
It's a strategy. An option in combat. I leave most standard critical hits at something like 5% (whatever the default of the system is) in order to accentuate the ways a player has of jacking that number up significantly. I have weapons that start with an extra 20% Critical Hit Rate. I have skills that increase an individuals' Critical Rate by 25, 50, 75, or even 100%. I have full party skills that increase it by 10, 20, 30, and 40%. I let them be a part of combat in which characters do exactly what the name implies: They hit an area that causes Critical Damage to an enemy. A vital organ. An artery. Something that handles fine motor skills. While I can't translate these effects well to an RPG Maker engine, like I would in D&D, I simply increase the rate. They are "aimed hits". They are precision strikes. Assassins trained in the art of killing will hit these areas easily and frequently. That's how I use Critical Strikes.
Do you give your players a way to avoid getting hit by a large critical strike from an enemy?
Yes, I do. There are certain armors that do this job. Even a few skills that do it. Some of my enemies will jack up their Critical Hit rates in combat and it would be a shame to not let the player protect themselves from these at all. I even have "Accessories" and "Relics" that can protect from Critical Hits. Though, the "Relics" come at a cost to something else for the highest protection bonuses.
Why do you have them in your game? Do you want to just make the player feel good when he gets lucky and lands one? Do you want to punish a player who got lazy about healing up and thought he could withstand an extra hit? Do they add anything meaningful to your combat system?
Why not have them? RPG Maker generally has a lot of wiggle room for interesting things to do in combat. The more options I have, the more I like to play around with them. The more I love to just include them. Why does everything have to strictly "Hit Hard"? Why is all combat DPS? I find that boring. I include options to improve your own chances of landing Critical Hits and to protect against enemies landing Critical Hits because they make combat feel less stagnant. Especially when I don't use traditional stats for dealing damage. Sure, you can equip the +102 Defense item, but it does little good when an enemy skill uses your Luck stat of 26 as your defense. That disparity between their attack power and whatever you're using for defense can be even more deadly if a foe can land a Critical Hit on you. Likewise, maybe you're using a weapon that doesn't add much in the way of Attack Power, but it adds a significant boost to your Agility stat that puts some of your skills over-the-top. Maybe, you can stack a Critical Hit skill on top of that damage your skills can do.
I use it because it's something else a player needs to watch out for. It's to minimize min/maxing. Or, in some cases, it helps aid in min/maxing and specializing a character or a player's play style.
I tend to not include critical strikes in my games - at least, not the way they're traditionally done. I usually have skills that automatically critically strike, either at the expensive of resources (mp/tp), based on a conditional (agility differential), or both. But, in the vast majority of rpg maker games I play, people use the traditional mechanics (+10% critical strike weapon) - and I'm curious what the design justification is.
The design justification for most anything in an RPG combat system is likely going to disappoint you... as it does me. And that justification is usually either, "I thought it might be cool" or "It's in there because I couldn't think of anything else to do". Namely, most people don't really plan out a combat system in RPGs. You'll find that's common across AAA games, Indie Games, and even down to our very own RPG Maker games. Most people don't think about it. The furthest they ever get is asking how you balance a combat system. Actually, you might find that's common for most aspects of any RPG. "It's in there because I think it's cool" or "I couldn't think of anything to do". Not a lot of people think, "Why is this in my game? What am I doing with it? What is its purpose? What am I hoping to accomplish by putting this into my game?". AAA developers are especially guilty of this.
To be fair, there are people who think about what their combat system will look like and why. But, the vast majority? Yeah... They don't think about it until they're already 30%+ done designing it.
My personal justification for anything in my game is that it adds something to the experience. Or, it changes how a player acts/reacts. I want X Effect, so I do Y Thing. I have a design philosophy of "What can the players do?". I then base all combat around everything the players can do. The more a player can do to "break the game" or "break combat", the more fun I get to have in turning those rules on their heads with weapons, armors, enemies, and enemy skills. So, the reason I have Critical Hits play a role in combat is two-fold. 1. Gives players more options in combat. 2. Gives me, as a dev, more options for messing with my players and trying to find ways to defeat them.