Perhaps just because it's better than never fasting at all?
The significance of Fridays, of course, is in commemoration of the Passion.
I grew up around three Catholic families and knew very well of their "meatless Fridays" but was too young at the time to understand, though their kids didn't understand any more than I did, they just hated fish and mac&cheese and really envied me during Lenten Season when I ate ice cream.