Ghusl for janaabah is an act of worship first and foremost
The believer should hasten to obey the command of Allah, may He be exalted, and the command of the Messenger (blessings and peace of Allah be upon him), and his obedience to the command should not be dependent on knowing the wisdom behind it. For many of the rulings of Islam we do not know the wisdom behind them, but we are certain that there is great wisdom behind them that Allah intended. The scholars call these rulings “rulings of unknown meaning” or “rulings on kinds of worship”, such as the number of rak‘ahs in prayer.
Some of the scholars are of the view that ghusl for janaabah comes under the heading of this type of rulings, the wisdom behind which is not known.
As-Suyooti said in al-Ashbaah wa’n-Nazaa’ir (638):
With regard to the rulings that have to do with rituals of worship…such as the ruling on minor and major impurity, these rulings have to do with rituals of worship, the wisdom behind which is not known; therefore they cannot be subject to rational thinking. Some of the scholars said: if it were not that they are rulings on rituals, emission of maniy (semen) which is taahir (pure) according to most of the scholars would not necessitate washing the entire body (ghusl), whilst urine and stools, which are naajis (impure) according to scholarly consensus, would not necessitate washing only part of it (wudoo’).
Based on that, it cannot be said that washing the head in the case of janaabah is because of secretions in the scalp. Rather we say that washing the head in the case of janaabah is part of the practice of the Prophet (blessings and peace of Allah be upon him), whom Allah commanded us to follow and take as an example. Allah, may He be exalted, says (interpretation of the meaning):
“Indeed in the Messenger of Allah (Muhammad SAW) you have a good example to follow for him who hopes in (the Meeting with) Allah and the Last Day and remembers Allah much.”
“Say (O Muhammad SAW to mankind): ‘If you (really) love Allah then follow me (i.e. accept Islamic Monotheism, follow the Quran and the Sunnah), Allah will love you and forgive you of your sins. And Allah is Oft-Forgiving, Most Merciful.’”
[Aal ‘Imraan 3:31].
The second type of rulings are those of which the meaning is known and the wisdom behind them is understood, such as the prohibition on khamr (intoxicants), so as to protect the mind, body and wealth, and the prohibition on dead meat and pork, because they are harmful, and so on.
Some of the scholars were of the view that the meaning of ghusl for janaabah may be understood on a rational basis; they said: The emission of maniy (semen) affects the body with some measure of weakness, laziness and exhaustion. Ghusl counters that and re-energises the body.
Ibn al-Qayyim (may Allah have mercy on him) said in I‘laam al-Muwaqqi‘een, 2/77-78:
Ghusl following emission of maniy is one of the most beneficial things for the body, heart and soul, and indeed to every part of the body, which is refreshed by doing ghusl. Ghusl counters the effects of emission of maniy; this is something that is well known from experience. Moreover, becoming junub makes one very tired but ghusl re-energizes one. Hence Abu Dharr said, when he did ghusl for janaabah: It is as if I have thrown a weight from me.
To sum up: this is something that everyone who has any common sense will understand and realize that ghusl after janaabah refreshes the body and the heart. As janaabah causes one’s heart and soul to become alienated, doing ghusl takes away that alienation. Prominent doctors have stated that doing ghusl after intercourse restores energy to the body; it is one of the most beneficial things for body and soul, and not doing it is harmful. The testimony of reason and sound human nature is sufficient to show that it is good. And Allah is the source of strength.
And Allah knows best.