Both. Most eye drops for glaucoma lower the eye's pressure. The lower the pressure, the slower the damage to the vision. The goal is to slow the damage to as close to a standstill as possible. The amount of pressure lowering needed to stop the vision loss is different for everyone and the eyes must be monitored closely by an eye doctor to ensure that no further vision loss is occurring.
Yes. Both. Eye drops slow the rate of deterioration by lowering the pressure, which stops one from going blind. Thus the key factor is testing the eye pressure and fields for the rate of change at the given pressure range (minus subjective variability).