In his blog, Eric Davis recently wrote, “Many years back I heard from somewhere that you should learn one layer above and below where you do most of your work. This was specifically for programming and implied your programming stack.”
I think the same rule can be applied to designers. For some designers, Photoshop is the primary layer of their work. But what's below it? For a web designer, HTML and CSS are below it. To be a better designer, I learned HTML and CSS. Above Photoshop is the user. Now that's a complicated layer. To understand the user, we need to study usability, accessibility, and even human psychology.
A designer that doesn't understand the layers above and below Photoshop doesn't want to be a designer, they want to be an artist. A designer who knows their stack, makes informed design choices from the start that result in sites that are go beyond aesthetics, they're functional to meet demanding goals.