I know that as per puranas like Shiva puran & Linga puran that , Lord Shiva had emerged as a fire pillar. Later on which became shivling. But are there any reasons as per Mahabharata that why it was worshipped? Does it mention about it?
Yes there are references to it in The Mahabharata. But there is no clear reason as to "why The Linga form".
One reference occurs in The Drona Parva, and specifically in the last chapters of that parva, after the Maha-Narayanastra was invoked by Aswattama (that section is also called as Narayanastra-Moksha parva).
After being spared by The Maha Narayanastra, the battle resumes and Arjuna prevails over Aswattama in a duel. Then he goes to the sage Vyasa (who happens to come there) asks about tall figure with matted hair, carrying a trident, who always goes a few feet ahead of his chariot, whose feet does not touch the ground. Arjuna describes this vision to the sage Vyasa, asking him who that person was. Arjuna tells the sage that it is from THIS tall person that myriad of weapons emerge killing all the foes, and it is THIS person who absorbs all the celestial weapons hurled by the enemies.
Sage Vyasa gives a lengthy response saying that figure is The Lord Sthanu, Sankara, Mahadeva. The sage says Lord Sankara is protecting Arjuna.
In one of those slokhas 124 - 126 of that chapter, The sage says the symbol / emblem is worshipped for the well-being of the world as well as a way of protecting / observing Brahmacharyam, which is worshiped by The Rishis, Gandharvas, and other celestials. The chapter also says Lord Shiva becomes very happy and pleased when HE is worshiped in the Linga form.