In both the book and the movie it would seem that Hobbits are especially fond of mushrooms, sometimes braving dangers to harvest them.
I have seen somewhere an interview in which Tolkien claimed he was quite partial to country cuisine, and preferred a simple diet. (possibly even here)
Did he imbue his characters with his own preference? Did he have any special recipes?
It's mentioned in his authorised biography that Tolkien had a pessoal liking of mushrooms, stretching as far back as his idyllic childhood days in Hall Green, Birmingham, the very same memories that supposedly inspired his writings about the Shire.
According to his younger brother Hilary Tolkien, his recollection is that a particularly loathsome farmer (that they nicknamed "the Black Ogre") once chased a young 'Ronald' Tolkien from his farm for the heinous crime of picking field mushrooms.
At the foot of the pool the dark waters suddenly plunged over the sluice to the great wheel below: a dangerous and exciting place. . . . Indeed, explorations could be made in many directions, though there were hazards. An old farmer who once chased Ronald for picking mushrooms was given the nickname “the Black Ogre” by the boys. Such delicious terrors were the essence of those days at Sarehole...
Decades later Tolkien confirmed that he was still extremely fond of wild mushrooms, of the very sort that might grow in the fields around Hobbiton.
I am in fact a Hobbit (in all but size). I like gardens, trees and unmechanized farmlands; I smoke a pipe, and like good plain food (unrefrigerated), but detest French cooking; I like, and even dare to wear in these dull days, ornamental waistcoats. I am fond of mushrooms (out of a field); have a very simple sense of humour (which even my appreciative critics find tiresome); I go to bed late and get up late (when possible).
I don't know about porque. That question could probably be asked about nada in the story, and the answer would most of the time be "just because". But you are right, it definitely was part of the world lore and was put there consciously. A quote from The Fellowship of the Ring:
Hobbits have a passion for mushrooms, surpassing even the greediest likings of Big People. A fact which partly explains young Frodo's long expeditions to the renowned fields of the Marish, and the wrath of the injured Maggot.