Every machine needs a home position

Datum, homing, and the reason a machine must be told where it is.

A machine that only counts its own movements from an unknown starting point has no way to know where it actually is, so before it can trust any position it commands, it has to visit one fixed, physical reference point it can find with certainty, however lost it might otherwise be.

The mechanism behind homing

An axis driven by a motor that only counts its own steps has no built-in idea of where zero is, since a count of steps only ever measures movement relative to wherever counting last started, and if that starting point itself was never confirmed, every position calculated from it is equally unconfirmed. Homing solves this by driving the axis toward one end of its travel until it triggers a fixed physical reference, commonly a small switch mounted at one extreme of the rail, and the moment that switch trips, the machine can say with certainty that the axis is at one exact, known position, regardless of anything it believed about its position beforehand. Every other position the machine will use for the rest of the job is then measured as a distance from that one confirmed point, effectively a working datum for everything the machine does afterwards, so the entire coordinate system the machine works from is only ever as trustworthy as that first physical contact. Some machines home every axis every time they start up, and others only when specifically asked, but either way the same principle holds, since no position calculated afterwards can be trusted until this one deliberate act of finding a fixed, physical reference has actually taken place.

The drawer comparison

A sliding drawer pushed firmly shut until it meets the back of the cabinet always ends up in exactly the same place, no matter how far open it happened to be beforehand or how carelessly it was pushed, because the back of the cabinet is a fixed physical stop rather than a guess, and reaching it removes any uncertainty about the drawer's position in one motion. Nudging the same drawer to a position judged by eye, however carefully done, never lands in quite the same place twice. A machine homing an axis is doing exactly what the drawer does against the back of the cabinet, driving deliberately into a fixed stop specifically because a fixed stop, unlike a position merely counted or estimated, cannot drift or be wrong, however long the machine has been running or however many moves it has made since it last knew for certain where it stood. Pushing the drawer only most of the way shut leaves exactly the same kind of uncertainty a machine has before it homes, a position that feels close enough by eye but carries none of the certainty the fixed stop would have given it.

The number that matters here

A homing switch typically only needs to be triggered to within a few hundredths of a millimetre of repeatability to be trustworthy, since it is establishing one single reference point rather than every point the machine will ever visit, and every position measured afterwards inherits whatever tiny uncertainty exists at that one physical contact, no more and no less than that. A switch that triggers a little differently depending on how fast the axis was travelling when it arrived introduces exactly that much uncertainty into every single measurement the machine takes for the rest of the job, which is why homing is usually done slowly and, on more careful machines, repeated a second time to confirm the first result before trusting it.

What follows from this

Because every commanded position is only ever a distance measured from that one home point, losing track of home partway through a job, through a missed step, a knock, or a power cut, invalidates every position calculated afterwards even though the motors themselves keep counting perfectly normally, which is why a machine that loses power mid-task is sent back to find its home point again before it is trusted to move anywhere meaningful, rather than simply resuming from wherever its last known count happened to leave it.