Yes, it is true that crying actually good for eyes. There are many reasons for this saying.
The tears could enable us to see. Tears are the lubricant in our eyes, which could lubricate our eyeball and eye lid. It can also help avoid dehydration of our mucous membranes. I can't imagine without this lubricant in our eyes. Moving or truing eyeball could be very uncomfortable. Secondly, tears could kill bacteria in eyes, which are so called the antibacteria and antivirul agent in our body. Whenever we got something into our eyes, tears can run out to wash our eyes. It contains lysozym, which could kill 90-95% of bacteria in our eyes.
Thirdly, tears could remove the toxins which are generated with your distress or grief. It can also cheer mood. After a good crying, you can feel much better, less stress, and relaxed. Anyway, crying is very good for your eyes and your health.