Most untrained grease gun operators do not wipe the grease gun coupling and grease nipple clean with a lint free cloth, before and after greasing each lubrication point. Bearings are precision devices manufactured in laboratory clean 'white rooms,' They will perform poorly and will rapidly fail if injected with dirt from a dirty grease gun coupler.
Tip from Lubrication for Industry 2nd Edition by Kenneth E Bannister (Industrial Press)