It is proven in the saheeh Sunnah that the first food offered to welcome the people of Paradise when they enter it will be “the caudate lobe of whale liver”. That is mentioned in the hadeeth of Thawbaan, the freed slave of the Messenger of Allah (blessings and peace of Allah be upon him), according to which one of the Jewish rabbis came to ask the Prophet (blessings and peace of Allah be upon him) about some matters to test him. In the hadeeth it says:
The Jewish man said: What will be presented to them first when they enter Paradise?
He said: “Caudate lobe of whale liver.”
He said: What food will be given to them after that?
He said: “The bull of Paradise, which used to graze along its edges, will be slaughtered for them.”
He said: What will their drink be?
He said: “From a spring there that is called Salsabeel.” …
Narrated by Muslim, 315
Imam an-Nawawi (may Allah have mercy on him) said: The words “What will be presented to them” refer to that with which a man is welcomed. Ibraaheem al-Halabi said: It is usually good quality fruits.
End quote from Sharh Muslim, 3/227
He also said:
This refers to a whale, according to scholarly consensus… As for the caudate lobe of the liver, this is a separate piece that is attached to the liver and it is the best part of it.
End quote from Sharh Muslim, 17/135-136
That is also proven in other saheeh hadeeths in as-Saheehayn and as-Sunan. We quoted this particular hadeeth because of what it contains of differentiating between the first food presented to the people of Paradise, which is the caudate lobe of fish liver, and the food that they will eat after that, which is the meat of the “bull of Paradise”.
We could not find anything to indicate the wisdom behind the caudate lobe of whale liver being the first food presented to the people of Paradise, but we believe that Allah’s wisdom is great and that Allah, may He be glorified and exalted, is the All-Knowing, Most Wise, and that He said of Himself (interpretation of the meaning):
“And your Lord creates whatsoever He wills and chooses, no choice have they (in any matter). Glorified be Allah, and exalted above all that they associate as partners (with Him)”
That should not prevent some of the scholars from trying to see the wisdom behind the choice of the caudate lobe of whale liver in particular. Some of them said that this is an indication of the end of this world, which is a transient abode, and moving to Paradise, which is an eternal abode, because the fish or whale is an aquatic animal which is indicative of the essence of life on earth, and the bull is a land animal which is indicative of tilling the soil and earning a living, so the people of Paradise are given these two things to eat to signal the end of this world and the beginning of the Hereafter.
See: Rooh al-Ma‘aani by al-Aloosi, 7/94
With regard to the benefits of the caudate lobe of fish liver, they are many, as has been discussed by doctors and nutrition specialists. Among these benefits are: lowering the level of cholesterol in the blood, reducing fat in the body, and lessening joint pain. It is also rich in vitamin D which has many benefits.
For more information on that, refer to specialist references in that field.
And Allah knows best.
(Source - Islam Q&A)