I heard that upper wing could increase stability, but why do fighters need it? Or are there some other reasons? Please specify.
One big reason is to make room for external stores such as fuel, weapons and electronics packs. A low-wing design must have longer and therefore heavier and bulkier undercarriage if ground access to the underwing hard points is to be adequate.
Also, the upper surface of a high wing is larger and cleaner, providing better lift over the fuselage and reducing the overall size of the wing.
Another trick which it allows is to create a waverider effect between the engines, providing supersonic lift with less drag.
The extra stability of the high wing position is unwelcome in a fighter and is typically countered by giving the wings a little anhedral, a slight downwards slope from root to tip.