Not all creatures in role playing games are equal, being more or less powerful. In games based on Dungeons & Dragons, this is primarily a function of level or Hit Dice.
In Dungeons & Dragons games, characters typically gain experience points for defeating enemies. Depending on edition, other activities can gain experience points. In first edition Advanced Dungeons & Dragons, most experience points (about 75%, if I remember correctly) actually came from recovering treasure, while second edition AD&D had each class gain bonus experience points based on class-related activities. Fighters from defeating enemies in combat, thieves from stealing treasure, wizards for researching spells and crafting magic items, and so on.
In Mutants & Masterminds, characters gain build points for their activities, but level advancement is more or less entirely at the GM’s discretion. M&M acknowledges the differing play styles that happen at different levels and tends to keep PCs in the middling level (roughly where I’d place the bread and butter Marvel heroes, actually). ‘Advancement’ mostly comes by broadening abilities: a PC gains new abilities, but doesn’t actually become ‘more powerful’ in a direct sense.
Hero System doesn’t use levels at all. Characters gain experience points that can be used to buy new abilities, buy off power limitations or personal disadvantages. These points can also be used to increase a power’s… power (Active Cost, how much you can do with the power), but is constrained by the power grade. In a Heroic-grade campaign, powers are limited to a certain Active Cost, combat values are constrained to a certain value, skill checks to another value, and normal gear is typically bought with gold rather than Character Points. In a Superheroic-grade campaign, the Active Cost, combat value, and skill checks can have higher values, and normal gear is paid for with Character Points. You can gain more points, but even if your Heroic character’s point total (character points plus experience points) is high enough to fit a higher grade campaign, you might still be in a Heroic-grade campaign — just very, very broadly capable within that grade.
Basic Roleplaying System from Chaosium also does not have levels, nor even experience points. A character has ranks in many skills and abilities. As these abilities are used, the GM may direct the player to make a mark next to the skill or ability. At a suitable time, the player rolls against the ability, and if the character fails the check (player rolls higher than the ability), the player can add a small amount to the skill. It’s easy to learn at lower skill levels, but becomes increasingly difficult as the skill increases.
GURPS is more or less the same as Hero System, for the purpose of this post. A PC starts with a certain number of points, there are campaign-specific constraints on how those points are spent, and there are no ‘levels’.
I’m going to keep levels. They give a very quick idea of how powerful a creature is. Depending on the game, they can also help ensure a character meets a certain baseline of ability. It can be easy to create a glass cannon or other character who is very capable in some way but missing some basic necessity. In a level-based system, it is fairly easy to set a baseline of ability on the level, then describe how the character deviates from that.
For instance, I might set a baseline attack bonus (comparable to the base attack bonus in D&D 3.x) equal to 3/4 of the creature’s Hit Dice. This is what a typical, active character or creature would have. Fighters are better at combat, so get higher bonus. Wizards are not as good at combat, so get a lower bonus. I could make the poor base attack bonus the baseline, but then ‘most’ creatures are better than this and combat-oriented characters are twice better… and I think that is a poor fit for baseline ability.
So: creatures advance by level or Hit Dice. These will have some baseline of ability, and the creature can be defined in part by how it deviates from this baseline.