Not only for the looks but also for a stronger core which is an amazing asset to have when performing compound movements.
How define your abs are determined by your body fat. The higher the body fat you have the less you see them.
Obliques
When you eat you don’t only eat 1 item you have sides that compliment what your eating, and obliques are the sides of the abs. They frame your abs but more importantly they are the pillars that make you rotate side to side.
My abs aren’t as defined as they used to be but they look decent due to my obliques. They make a huge difference in how your entire mid-section will appear.