Praise be to Allah, and peace and blessings be upon the Messenger of Allah.
Allah تعالى has made this worldly life a place of trial and testing. One of the greatest purposes of this is that the servant believes in what Allah has informed him about concerning the unseen, without seeing it with his physical senses.
If Paradise and Hell were part of the visible world, and people could see them directly, there would be no longer be room for testing or distinguishing between the believer and the disbeliever. Seeing Hell and Paradise and directly witnessing their reality would compel a person to believe.
For this reason, Allah تعالى says:
﴿الَّذِينَ يُؤْمِنُونَ بِالْغَيْبِ﴾ [Al-Baqarah: 3]
“Those who believe in the unseen.”
Belief in the unseen is therefore one of the greatest distinguishing characteristics of faith.
The Islamic texts also indicate that faith does not benefit a person when certain great signs appear that take him out of the state of the unseen and the period of testing, if he had not believed beforehand. Allah تعالى says:
﴿يَوْمَ يَأْتِي بَعْضُ آيَاتِ رَبِّكَ لَا يَنْفَعُ نَفْسًا إِيمَانُهَا لَمْ تَكُنْ آمَنَتْ مِنْ قَبْلُ﴾ [Al-An'am: 158]
“On the Day when some of the signs of your Lord come, no soul will benefit from its faith if it had not believed before.”
One example of this is the rising of the sun from the west. When people see it, they will believe, but at that point the faith of one who had not believed beforehand will not benefit him.
If the appearance of some of the signs of the Hour prevents new faith from being beneficial, then seeing Paradise and Hell directly with one's physical senses in this world would be even more effective in removing the meaning of the test.
Yes, a person can imagine some of what Allah and His Messenger ﷺ have told us about them. The Qur'an and Sunnah have described Paradise with its rivers, palaces, food, drink, spouses, and bliss, and Hell has been described with its various forms of punishment.
However, no matter how much a person imagines the bliss of Paradise, its reality is far greater than anything he could possibly conceive. Allah تعالى said in the sacred hadith:
“I have prepared for My righteous servants that which no eye has seen, no ear has heard, and has never occurred to the heart of a human being.” Narrated by al-Bukhari.
The descriptions of Paradise found in the texts bring its meanings closer to people by using things familiar to them in this world. But its true reality, and the bliss Allah has prepared within it, are beyond anything human beings have known or imagined.
Likewise, the descriptions of Hell that have been transmitted only reveal something of its reality and punishment. As for its true nature and details, they are among the matters of the unseen that a human being cannot fully comprehend in this world.
Thus, the purpose is for the servant to remain a believer in the unseen, to believe what Allah and His Messenger have informed him, and to work for what comes after death, while knowing that what Allah has prepared in the Hereafter is far greater than what the descriptions of this world can encompass.
And Allah تعالى knows best.