Few maintenance items are recommended as frequently as chain lubrication, and the interval surprises riders coming from cars. The reason lies in what a chain is and where it lives.
A chain is a row of plain bearings
Each link contains a pin rotating inside a bush, and there are well over a hundred of these joints in a typical chain.
Every joint articulates twice per revolution of the sprocket, once entering and once leaving, so a single ride cycles each bearing an enormous number of times.
All of this happens under the full drive torque of the engine, concentrated into a contact area smaller than the head of a pin.
Modern chains lubricate the joint internally
Sealed chains hold grease inside each joint behind rubber rings, so the lubricant that matters most is factory-installed and cannot be replenished.
Applied lubricant protects the rings themselves and the rollers that contact the sprocket teeth, rather than reaching the pins.
This is why chain life depends heavily on keeping those seals intact, and why aggressive solvents and pressure washing shorten a chain's life more than neglect of lubrication does.
The chain runs in the worst possible environment
Unlike almost every other loaded component on a vehicle, the chain is exposed to rain, road salt, grit and whatever the front tyre throws at it.
Grit that adheres to a lubricated chain becomes an abrasive paste that works at the seals and at the roller and sprocket interface.
Cleaning is therefore at least as important as lubricating, and lubricant applied over contamination can be worse than applying nothing.
Wear shows up as stretch, which is not stretch
A worn chain measures longer, but the metal has not extended. Material has been removed from the pins and bushes, so each joint has more play.
Multiplied across a hundred joints, that play becomes several millimetres of apparent lengthening, which is what a wear gauge measures.
A stretched chain no longer matches the sprocket pitch, so load concentrates on fewer teeth and the sprockets wear rapidly, which is why the set is replaced together.
Adjustment protects the rest of the drivetrain
Chain tension changes as the suspension moves, tightening as the swingarm passes through a particular part of its travel.
Set too tight, the chain loads the gearbox output bearing and the swingarm bearings every time the suspension compresses, and those are expensive components.
Set too loose, it can jump a sprocket tooth under a sudden load or strike the swingarm, which is why the specified free play is a range rather than a maximum.