If I'm working on user research I need to understand the people (user groups) who use the product and how they use it. If I'm working on interaction design, I need to understand how each interaction takes place, what problem it's solving, what that means for the user, and how it effects anything else. If I'm working on visuals/graphics, I need to design the product/features/interactions in an aesthetically pleasing way that still targets the user groups.
So UX will guide you towards what features are important and ways of exposing those features so the end user has the best experience. UX also tends to focus on testing assumptions.
Visual design will guide you towards making sure the elements on the page are aestheticly pleasing and that they communicate what is trying to communicated. Google is famous for testing it's shade of blue, but, IMO, great design can't be tested, but rather comes from a designer understanding her audience and the story she is trying to tell.
Don't get me wrong, usability testing is incredibly important. Every programmer can tell you how much they learned the first time they watched an uncoached newbie try out their software. And even with those paradigm-shifting design changes, you can and should test that the results are really an improvement. But you can't make big important changes by measuring the impact of every little detail.
That is the sort of skill it takes to be a great designer IMHO.