Undoubtedly, wudhu possesses two manifest benefits - the medical benefit and the ethical and spiritual one.  From the medical point of view, washing of the face and hands - and that too fives times a day or at the very least, three times a day – has an appreciable influence as far as the cleanliness of the body is concerned.

 Wiping the head and the exterior portion of the feet - the condition here being that the water reaches the hair and the skin - becomes reason that we keep these portions clean too.  And as we shall allude later when discussing the philosophy of ghusl, the contact of water with skin has a special effect in achieving the equilibrium of the sympathetic and parasympathetic nerves of the body.

 
As for the ethical and spiritual aspect, since it is performed for God and with the intention of pleasing Him, it possesses an educative influence; especially since its implicit meaning - from head to toe I strive to obey You - serves to corroborate this ethical and spiritual philosophy.

In a tradition, Imam A"li Ibn Musa al-Ridha (peace be upon him) says:
 The reason that the order for (the performance of) wudhu has been issued, and (the reason) that the acts of worship should commence by it is that when the servants stand before God and converse with Him, they should be clean, away from uncleanness and pollution, and act in accordance with His orders.  Apart from this, wudhu eliminates drowsiness and lethargy from man so that the heart can acquire the purity and luminosity for standing in the presence of God.

When we explain the philosophy of ghusl, the philosophy of wudhu shall become more illustrated.

 
PHILOSOPHY OF GHUSL

Some people question: Why does Islam order a person in the state of ‘janabat’  to wash his entire body whereas it is only a particular organ that becomes unclean?  Is there a difference between urinal emission and seminal discharge so as to necessitate the washing of only the organ, in the former, but the entire body, in the latter?

There are two answers to this question - one brief and the other comprehensive.

The brief answer is that the discharge of semen from the human body is not an act that is restricted to just one part of the body (unlike urine and other body wastes), a claim which is substantiated by the fact that the effect of the discharge becomes manifest on the entire body – subsequent to a discharge, all the cells of the body slip into a characteristic listlessness; this itself being an indication of its effect on all the parts of the body.

Explanation:

Studies conducted by the scientists reveal that within the human body there exist two vegetative nervous networks which control and regulate all the activities of the body.  These two nervous networks - the Sympathetic nerves and the Para-sympathetic nerves - are spread out throughout the body and around all the internal and external systems and tracts. The function of the Sympathetic nerves is to accelerate and stimulate the activities of the various tracts of the body, whereas the Parasympathetic nerves function to decelerate and diminish them.  In effect, one plays the role of the accelerator of an automobile while the other plays the role of the brakes; with a balanced functioning of these two sets of nerves, the systems of the body work in a balanced and normal manner.

At times certain occurrences in the body disrupt this balance and equilibrium - one of these being the issue of ‘orgasm’, which is usually contemporaneous with a seminal discharge.

In such cases, the Parasympathetic nerves (the decelerating nerves) tend to take a lead over the Sympathetic nerves and consequently disrupt the equilibrium, negatively.

It has also been established that amongst the things that could force the Sympathetic nerves into activity and re-establish the lost equilibrium is contact of water with the body, and since the effect of orgasm is noticeably felt on all parts of the body and the equilibrium existing between these two sets of nerves is disrupted all over the body, hence it has been ordered that after sexual intercourse or seminal discharge the entire body should be washed with water so that as a result of its regenerative effect, equilibrium is once again established between these two sets of nerves, all over the body.

Of course, the benefits of ghusl are not just confined to the above for, in addition to this, it is also a worship whose ethical influence cannot be denied.  And it is for this reason that if the body is washed without the intention of seeking His pleasure and in compliance with His orders, the ghusl would be deemed to be incorrect.  In reality, seminal discharge or sexual intercourse tends to affect the soul as well as the body - the soul gets drawn towards material pleasures, while the body is overcome by listlessness and stagnation.  The ghusl of janabat, which is a washing of the body and, due to its being performed in compliance with God’s orders and with the intention of seeking His pleasure, also that of the soul, exercises a two-fold effect upon the soul and the body - leading the soul towards God and spirituality while at the same time leading the body towards cleanliness, liveliness and activity.

In addition to the above, the obligation of the ghusl of janabat is an Islamic compulsion for maintaining cleanliness of the body and observance of hygiene, throughout the life.  This is because there are numerous individuals, who are neglectful of their cleanliness and hygiene, but this Islamic ruling forces them to wash themselves at regular intervals and keep themselves clean.  And this is not specific to the people of the past eras, for even in our times there are numerous such individuals, who, for various reasons, tend to be neglectful of their cleanliness and hygiene (However, this is a general and universal rule and includes even one, who has recently washed his body).

The abovementioned three aspects clearly illustrate why the ghusl ought to be performed and the entire body washed after a seminal discharge (sleeping or awake) and similarly, after sexual intercourse (even if not accompanied by a seminal discharge).

Source: Tebyan