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.”

[al-Ahzaab 33:21] 

“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.

End quote 

And Allah knows best.