The case for fixed level ups in Fire Emblem
When you are evaluating the viability of a unit in Fire Emblem, there are many factors you must consider: How long they are available, their weapon ranks, their class, etc… However, if we were to oversimplify things, we could just talk about base stats and stat growths.
Base stats are the stats that a characters starts with. They are important to determine the immediate usefulness of a character. A character with very high base stats will be useful right away. If their base stats are high considering their level, it actually helps their long-term viability as well. If you get a level 10 character with nearly maxed strength and speed, it does not matter if they do not level these stats often; they are already viable. These units tend to be reliable, and some players would argue that choosing reliable units is the way to go, as they are less tedious to make use of. They would argue that a character with low base stats could technically become good but at the cost of a lot of investment put into them, and it is not necessarily always going to work out in your favour.
Base growth determines a character’s chance of getting an increase in one of their stats. A 70% strength growth means that most of the time, they would be likely to gain more strength, but that is a 70% RNG roll every single level up. That means that it would be technically possible to roll very poorly on some runs, and great in others. High growth units that have many levels to go have a lot of potential, but potential does not always lead to results. I have seen playthroughs of Path of Radiance where Ike, a unit that tends to have a very high strength growth, where Ike had gotten only 4 strength level ups during the entire game. It is very unlikely, but it can happen.
Showing the variance:
This amount of variance has many implications, game design-wise. On one hand, it means that every playthrough will be different. Perhaps some characters will be very RNG-blessed and simply carry your run on one playthrough, while in another run, your MVP might be completely different. It leads to a lot of variety, where every playthrough is different than the last. However, this also means that in terms of game-balance, the developers must consider the RNG as a factor when designing maps.
Imagine that you make a boss with very high defense that only your main character can defeat. However, your character just didn’t level strength enough, and just does 0 damage to said boss. You are now locked in a no-win situation where you just cannot continue your run, no-matter what. The game has to be designed around multiple scenarios, where they must consider all sorts of results. This is why they tend to give very powerful units and tools to the player at various points in the games, in order to allow the player to progress in the worst possible circumstances. In FE 7, you get Marcus to help newbies, and Isadora joins much later to help in the case you might have lost Marcus. Then Athos is given to you on the last chapter in case you have nothing to defeat the final boss with.
However, in a game where you have less room for random variance, you can create far more intentional map-design. In Shadows of Valentia, the DLC maps were centered on a selected few units that had fixed levels and equipment. It led me to more effectively trade weapons and items between characters in clutch moments, or using a character’s abilities to their fullest. In the Cindered Shadows DLC for Three Houses, having more limited room to grow let them make maps taking advantage of the abilities of the set characters. While you could level up, the amount of levels you could obtain between the start and end of the DLC and the start of the main story to its end-game was much lower. This led to some creative map ideas that the main game could not have pulled off. In some cases, the damage my characters would do would be flush with the enemy’s HP on hard mode. Because the game knew you have to have a flier, one map made use of that fact by putting forests in front of your objective.
The fact is that with a fixed-level-up system, the developers would have a far better idea as to what the average stats of a player’s characters on a given map would be, and they could balance levels around that, adding a bit of a puzzle element to the maps.
Now, I am not saying that this is what they should do, but I very much enjoy those smaller Fire Emblem experiences that make stat variations less of a factor. I am aware that this has become a staple of Fire Emblem, but if anyone were to make a Fire Emblem-type game with a different spin, it may be an option to consider.