In fact it is a solution to a complex operational problem. Fixed eyes are effective in simple organisms, but impose a significant impediment to survival for animals that must contend with the challenge of predators or agile prey. The development of eyes able to move within an eye socket marked a significant improvement and demanded, in turn, a lubricant able to bathe delicate optical equipment and ease the friction of motion. The lubricant itself needed special properties, low viscosity so as not to entrap debris, low production cost so as not to consume too large a portion of the organisms food intake, and low opacity so as not to interfere with systems original visual purpose. High on the list of requirements too was the necessity that the lubricant should not harbor bacteria.
Tears therefore contain a specific agent that attacks just bacteria, destroying their cell walls.
The agent which nature developed for this purpose is lysozyme. It does not effect the delicate membranes of the eye or the eye socket. Instead, it is able to recognize at the molecular level a component of a bacterium's cell wall and, having keyed onto this element, break down the wall destroying the foreign organism. Lysozyme is an elegant solution to a thorny operational problem and it is just one of the 10,000 different proteins which make up about half the dry weight of a typical mammalian cell. These special molecular solutions in the form of molecules are among the molecules of life.