Considering the reasons and purposes of the laws...
Considering the reasons and purposes of the laws, the shari'ah can be divided into four categories: The laws whose reasons and purposes are self evident : For example, helping the needy is highly recommended; killing is forbidden; lying is evil; paying taxes like khums and zakat is obligatory.
One does not need any expertise or extraordinary intelligence to know that helping the needy is good, paying taxes is necessary for preserving the financial equilibrium in the society; and that killing and lying is evil. The laws whose reasons and purposes have been explained in the Qur'an and hadith : For example, intoxicants are forbidden, interest is prohibited, fasting in Ramadhan is obligatory and prayers are a must.
The Quran and hadith have said that intoxicant is one of the main causes of evil because an intoxicated person is no longer in control of himself. Although it took the world a long time and a bitter experience to realize the wide-spread harm of drunkenness, Islam declared its harm and evil fourteen centuries ago by saying "its sin is greater than its profit." (Surah al-Baqara, 2:219) Interest is forbidden in Islam. The Qur' an and hadith have explained the harm of interest.
Interest leads to destruction of the poor section of the society, and all wealth gravitates towards the already wealthy group.[^1] Fasting is a physical and spiritual training which brings the servants of Allah (SWT) nearer to Him and makes them more obedient to the shari’ah. Prayers is a means of expressing our gratitude to Allah (SWT): “O you who believe!
Eat of the good things that We have provided you with and thank Allah.” (Surah al-Baqara, 2:172); it is an important way of achieving peace of mind*:* “surely by Allah's remembrance are the hearts set at rest.” (Surah ar-Ra’ad, 13:28); and it is also a very effective method of making the believer more obedient to the laws of Islam*:* “ surely the prayer keeps (one) away from indecency and evil.” (Surah al-Ankaboot, 29:45) There are many ahadith of our Imams explaining the reasons and purposes of many laws of the shari'ah.
Shaykh as-Saduq, the famous Shi'ite scholar, has collected many of these ahadith in his ‘Ilalu ‘sh-Sharaya’. The laws whose reasons and purposes have not been explained in the Qur'an or hadith, but the rising horizon of human knowledge have helped in understanding their purpose and usefulness.