There are 3 purposes of sexual intimacy in Islam.
1. It is mandatory for the conservation of human race and continuity of its kind. It has this divine purpose to show gratitude to the creator.
2. The congested semen or body fluids inside the body can cause serious illnesses and hence has to be ejected out.
3. To fulfill ones desire of sexual pleasure and enjoyment of this blessing.
Apart form these purposes, it is good for human health and yields essential health benefits.
Semen is rich in prostaglandins, which are harmone like fatty acids. These are found in whole of the body but vastly in semen. These are primary nutrients that affect the essential and normal body processes which include blood presure, metabolism, body temperature etc. Hence understanding its effects on body, this important water must be wisely used.
This means to wisely value the reasons behind dispensing of this precious water of life for either conceiving a child, or emitting it through lawful sexual intercourse upon congestion.
Congested semen which are retained for long periods of time inside the body may effect it negatively and may cause various ailments including obsession, lunacy and even insanity. Engaging in lawful sexual intercourse may aid in the recovery from such illnesses.
Moreover, congestion of semen for extra long periods can cause its corruption and may turn it into harmful toxin that a human body cannot easily dispose off. However, sometimes,nature produces a spontaneous and involuntary emission of excess semen, usually during sleep, and without sexual intercourse.
A person must not abstain from having lawful sexual intercourse, for a water well drains out if its water if it's not used regularly.
Imam Muhammad ibn Zakariyya once said:
"Abstaining from sexual intercourse for an extended period weakens one’s nervous system, can cause obstruction of the urethra, and shrinks the penis."
He added when he observed some people who vowed temporary abstention from sexual intercourse, that their sexual energy diminished, they suffered general weakness of their bodies, became doleful, lost their desire, and their digestive system became corrupt.
All these benefits are that kf lawful sexual intercourse. Unlawful intercourse leads to destruction and wrath of Allah taala.
Other benefits of lawful sexual intercourse include protection from unlawful things including looking at haraam and preserving ones chastity. With it a person is able to control his desire from unlawful and haraam and protects his spouse as well. Imam Ahmad alluding to abstention from sex, once said:
"I exercise patience when fasting from food and drink, though it is still difficult."
He also reported in his collection of correct prophetic traditions that Allah’s Messenger (SAW) encouraged the believers to get married, saying:
"Get married, and conceive many children for I shall take pride in your number on the day of judgment."
Ibn Abbass (RA) narrated that Allah’s Messenger (SAW) said:
"I get married, eat meat, sleep, stand up in night prayers, fast, and break my fast. Whoever disdains from my traditions (Sunnah) is not one of my followers."
He (SAW) also said:
"O young men whoever among you has the means to establish a family, he should get married, for marriage preserves the chastity of one’s eyes and sexual organ, and whoever cannot afford to establish a family, he must fast from desiring sex, for abstention in that case will protect him from sin."
Ibn Abbass (RA) one said:
"We recognize that marriage is the best solution for two people who are in love."
It is also narrated in the two collections of correct prophetic traditions that Allah’s Messenger (SAW) said:
Allah’s Messenger (SAW) also encouraged people to choose the young and fertile. On this subject, Mu’qal bin Yasar related that a man said to Allah’s Messenger (SAW):
"I found a beautiful woman from a noble family but she cannot bear children, should I marry her? Allah’s Messenger (SAW) replied, "Nay." The man came back and asked a third time and asked the same question, Allah’s Messenger (SAW) turned to his companions and said: "Choose in your wives the fertile and the affectionate, for I shall take pride in your number on the day of judgment."
And Allah knows best.