Lets break this question into parts and answer them one by one.
Do tears really save us from harm?
Well, yes. Tears, mucus and saliva contain an enzyme lysozyme that breaks down the cell wall of many bacteria. Those that are not killed immediately are trapped in mucus and swallowed.1
You can, obviously, search on google for another reference if you do not believe this one.
Or are they just the reflex shown?
It depends on what type of tears you are talking about. Tears are divided into 3 categories:
- Basal tears
- Reflex tears
- Psychic tears
In "reflex action" part, the cause is reflex tears. Reflex tears result from irritation of the eye by foreign particles, or from the presence of irritant substances in the eye's environment, which trigger TRP (Transient Receptor Potential) channels in the ophthalmic nerve. It can also occur with bright light and hot or peppery stimuli to the tongue and mouth... These reflex tears attempt to wash out irritants that may have come into contact with the eye.
These are, of course, different from those tears which "suddenly start flowing out of our eyes when we find our long lost friend or when someone unexpectedly decides to break up with us". These are psychic tears i.e. increased tearing due to strong emotional stress, pleasure, anger, suffering, mourning, or physical pain. This practice is not restricted to negative emotions; many people cry when extremely happy such as during times of intense humour and laughter.2
But the actual relation of tears with emotions can be summarized as:
- There is an area of your brain specifically to deal with your emotions, called the limbic system (specifically the part of it called the hypothalamus), which is hard-wired into your autonomic nervous system (that’s the part you don’t have any control over).
- This system, via a neurotransmitter called acetylcholine, has a degree of control over the lacrimal ‘tear’ system; and it is this tiny molecule which then stimulates tear production.
- So in short, your emotional reaction to the break-up triggers your nervous system, which in turn, orders your tear-producing system to activate.3
Moving on to next part:
crying causes endorphin release...Are tears the only way to accomplish this?
Certainly not. Even when you exercise, your body releases endorphins. These endorphins interact with the receptors in your brain that reduce your perception of pain. So, tears are not the only way to accomplish this.4
At last, I would answer this part
What biological purpose do tears serve for the body?
as tears are necessary for the continued health of the ocular surface. Normal constituents include water, mucin, and lipids, electrolytes, non-electrolytes, and proteins. Retinoids are also important for ocular health and prealbumin may be a carrier for vitamin A in the tears to supply corneal epithelium with its requirements. Changes in tear constituents may cause certain ocular disorders.5
It is important to know here that by the above part, I meant basal tears and not psychic ones. Basal tears lubricate the eye, and help to keep it clear of dust. Tear fluid contains water, mucin, lipids, lysozyme, lactoferrin, lipocalin, lacritin, immunoglobulins, glucose, urea, sodium, and potassium. Some of the substances in lacrimal fluid (such as lysozyme) fight against bacterial infection as a part of the immune system. Lysozyme does this by dissolving a layer in the outer coating, called peptidoglycan, of certain bacteria. It is a typical body fluid with a salt content similar to blood plasma.
EDIT 1: Now, as you asked in the comments
what if we don't maintain our ocular surface?
I can tell you a very common example of dry eye syndrome. Dry eye syndrome is a common tears and ocular surface multifactorial disease, described by changes in the ocular surface epithelia related to reduced tears quantity and ocular surface sensitivity, leading to inflammatory reaction. It is believed that decrease in the tear volume on the corneal and conjunctival surface caused by either a decreased tear secretion or accelerated evaporation plays the main role. The clinical features of dry eyes include ocular discomfort, feeling of dryness, feeling of eye fatigue, hyperemia, kerato-conjunctival epithelial disorders and abnormalities of vision.6
EDIT 2: When it comes to function of psychic tears, then how could one leave talking about why we feel better after crying (except headache). It is now proven that psychic tears contain a higher amount of hormones than basal or reflex tears. See this article7:
Emotional tears are composed of more protein-based hormones, such as prolactin, adrenocorticotropic, and leucine enkephalin (a natural pain killer), which is suggested to be the mechanism behind the experience of crying from emotion making an individual feel better.
Though these hormones do not have any direct relation with emotions (excluding prolactin which counteracts the effects of dopamine), they can be related to emotions as long as another biological reason for their secretion through tears is discovered.
Bonus: From the same article 7, here is a very unusual function of tears i.e. controlling arousal! See this paragraph:
On a study conducted by the Weizmann Institute of Science in Rehovot, Israel, emotional tears from women have been found to reduce sexual arousal in men. Also, emotional tears are made up of a different chemical component than those evoked by eye irritants and can relay chemical messages to others. The change in sex drive could be attributed to a drop in testosterone provoked by the tear chemicals, reducing aggression. In the animal world, it has been found that some blind mole rats rub tears all over their bodies as a strategy to keep aggressive mole rats away.
This way, we can soon have a long list of functions. I hope these many functions are enough to answer your question.
1: The body’s first line of defence
2: Tears - Wikipedia
3: Why do we cry? The science of tears
4: Exercise and Depression
5: Eyelid secretions and the prevention and production of disease
6: Recent developments on dry eye disease treatment compounds
7: Chemicals in tears - Wikipedia