This post's somewhat of an extension of my previous post about encouraging cheese
All games should have a set of formal incentives/rules/categories that reward or recognize desirable playstyles and punish undesirable ones. And they should flesh them out somewhat. This might come off as obvious, pointless, trivial but bear with me.
Games are systems with their own internal logic. Usually this logic isn't obvious because the games have some contradictory design that pulls them in different directions - maybe in some ways they encourage you to build speed, but in other ways reward playing it safe. The more pressure you apply to a game through higher difficulty or optimization (score runs, speedruns), the more those elements will fight each other, with an eventual winner.
This is further complicated by the "soft" layer of player psychology. Some good things are too boring for players to do (for example farming points via repetitive actions), which creates a kind of soft pressure against doing them.
This layer of soft risk vs reward usually obfuscates the internal logic of a game. The problem is that this layer just doesn't scale properly because of how volatile and conditional the player's feelings are.
Let's take Contra - the internal logic fully allows you to slowly scroll the screen one step at a time (called stepping, or edging 🙃) , spawning enemies 1 by 1 or in very small groups and killing them from a safe distance. In the game's eyes you played it as well as a speedrunner if you do this.
Because Contra games make fast playstyles easy & fun, almost nobody is going to play these games this way, especially once they have a basic familiarity with the levels.
But now imagine turning the difficulty knob up, gradually making the game harder - more enemies, more bullets, more precise shots, etc. Eventually, for most people, the game will hit a point where its internal logic overrides any soft psychological incentives. Players will be forced into a boring playstyle because they feel like the risk isn't worth the non-existent reward. Without formal systems, dealing with that risk & challenge will feel arbitrary.
The fact that it has to pass your personal threshold for diffiuclty and arbitrariness makes this issue sneak up on you. You can handwave away contradictions in the game's logic saying something is too boring to bother with, until it hits you like a truck, after which it's too late - the game has no safety net to prevent this issue. And it probably will sneak up on you sooner or later, if it hasn't already.
Scoring systems, stage ranks, leaderboards, performance-based unlockables, even text pop ups calling the player your little pogchamp are all relatively low effort ways to create a safety net.
Another BIG benefit of focusing on the formal is also that it lets you get away from retroactively looking at player psychology, and into thinking more proactively about what kind of artistic experiences you want to create. Players don't know what they want, so forcing them to engage will often sell them on playstyles that they thought they would hate and help develop their tastes. The player's perception about whether or not a type of challenge is rewarding or arbitrary is heavily conventions-driven, after all.
