An accident of stages
The checkpoint was not invented so much as inherited. Early arcade games were stage-based and gave the player multiple lives, and the obvious behaviour on death was to restart the current stage rather than the whole game — Galaxian returns you to the stage you were on. That is not a designed mercy; it is the least surprising thing to do given a structure of discrete stages and a life counter, and it produced the checkpoint as a by-product.
In early games checkpoints were invisible. There was no marker, no flag, no sound — the only way a player learned where the checkpoints were was by dying and observing where they reappeared. The system was communicating its rules exclusively through failure, which is a strange thing to say about a mechanism whose entire job is managing failure, and it meant that understanding a game's checkpoint structure was itself something you had to die to learn.
Setting the price of death
Once checkpoints become deliberate, they become the primary instrument for setting difficulty — more so than enemy health, damage numbers, or any of the things usually labelled as difficulty. The reason is that a game's difficulty, as experienced, is not the probability of failing a challenge. It is the expected cost of failing it. A brutally hard sequence with a checkpoint immediately before it is an enjoyable puzzle you retry instantly. The same sequence with a checkpoint ten minutes back is a different game entirely, and a considerably worse one, without a single enemy having changed.
This is why checkpoint placement is where designers reveal what they actually think the game is about. Place them densely and you are saying the interesting part is the challenge itself, and you want the player attempting it repeatedly. Place them sparsely and you are saying the interesting part is the tension of a long run, and you want the player to feel the accumulated weight of progress they could lose. Both are legitimate. They produce almost unrelated experiences from identical content.
Mandatory, optional, invisible
The mechanics are simple enough to enumerate. Checkpoints can be mandatory — activated automatically when passed — or optional, requiring the player to do something to claim them. The game stores which was last activated and restarts the player there rather than at the level's start. Modern games mostly auto-save at these points, and in many cases the checkpoint has merged with the save system entirely, becoming a fixed location where progress is written and death returns you.
The optional checkpoint is the most interesting of these, because it hands the decision to the player and thereby turns a difficulty setting into a gameplay choice. Do you detour to activate the marker, spending time and possibly risk, or push on and gamble that you will not need it? Survival horror built substantial tension out of exactly this by making the save itself a scarce resource, which converts the question "should I save?" into a real decision with a cost. The moment a checkpoint is something you might not take, it stops being infrastructure and becomes part of the game.
What the invisible ones taught
The early invisible checkpoint has a quality worth recovering. Because you learned its position only by dying, the geography of failure was something you built up over attempts — a mental map of the level marked not by where things were but by where you would resume. Players developed an intuition for it, and part of what mastering an arcade game meant was knowing the checkpoint structure well enough to reason about risk.
Modern design has largely eliminated that, and mostly for good reason: a visible checkpoint that clearly announces itself lets the player reason about risk without paying for the information in deaths. But the trade is real. A game that tells you exactly where you will resume has converted an unknown into a known and removed a source of tension, and the ones that deliberately withhold it — or make you choose whether to claim it — are exploiting a mechanism the earliest games stumbled into by accident. Where the game puts you after it kills you was never a technical detail. It is the game telling you how much it thinks your time is worth.