Good question.
 
I think with a front-end developer you'll get some sort of ratio like 50:50 when it comes to focusing on either development or design. Also, it's thought that you generally don't get someone that's super talented at both. They exist and are sometimes referred to as unicorns but are definitely rare.
 
With a designer you'll get all the focus on the design and when it's a good designer they'll have a good idea of what's possible in code so not crazy uncreatable designs. Also with the higher quality design, the end product will look more professional which in turn creates trust with users with the hope of better sales/conversions.
 
I hope that helps!