what does increasing the parasympathetic tone in the eye result in?

Parasympathetic stimulation of eye causes miosis of the pupil (constriction... makes it smaller). Moreover, the oculomotor nerve also carries parasympathetic fibers to the ciliary ganglion of the eye. From the ciliary muscle, postganglionic fibers eventually reach the ciliary muscle to help in accommodation of the lens. This is an extremely simplified version. About your question on intraocular pressure, yes it is slightly beneficial due to the fact that it may decrease intraocular pressure. However, it's coupled with an increase of aqueous humor. Although it may decrease intraocular pressure, it is not sufficient enough to help a patient with glaucoma for example.