I'll tell you why. The recent style introduced into the fashion world of wearing a sweater over a dress is actually a lot cuter than it sounds; mainly because throwing a single toned sweater over a patterned dress is actually adorable and catches your eye.
Also, mix and matching different shades and tones of your sweater and dress makes for a really calm and surreal feel to your outfit, like shown below.
I think just from those few examples, you can tell this is a really cute idea and really easy to pull off.
Also, it's very helpful if you're a dress person like I am; if I want to wear a dress but it's a chilly day, I'll just throw on a thick sweater and a belt along with some tights and cozy shoes on the bottom and I'm good to go!
Try it sometime, I promise you won't regret it!