The purpose of the valve stem seal is to prevent the oil from the cylinder head to enter the combustion chamber. Since the valve stem is moving up and down by the camshaft, the moving stem has a rubber seal against the stationary cylinder head.
If the clearance between the stem seal and the valve guide is too great or if the seal has been damaged during installation, then increase quantities of oil is leaked past the seal. The clearance can be too great if the seal is worn or damaged, or the valve guide is worn or damaged allowing too much play in the valve movement.
A refurb of a cylinder head will among other things include the replacement of the valve stem seal as the rubber material hardens over time and the spring loses tension.