Question: Why is it some employers want to do video calls, such as Skype, for interviews instead of just a phone call? The only reason I can think of is so they know how you look, but what difference does that make?

Answer: 
Lots of reasons. First, they want to make sure you aren't cheating on the questions by having someone else feeding you the answers or looking them up on Google.

They want to see your body language to assess your reactions to the questions beyond just your words. 

There are some unethical people who do the phone interviews and then are not the people who show up when they get hired. So they want to see you to see if you are actually thhe person they hired when you get there. This is especially true if the hiring offical has been burned in the past by having a very knowldgeable phone interview result in a totally incompetent person showing up for work.

And depending on the job, appearance might be important as you might have customer contact and they want to ensure you are someone they would want to see as the face of the company.

Will some people be assessing you on irrelvant factors like race or weight? Yes. But those same type of people may be assessing you on other irrelvant factors on a phone call like your name or your accent.
Video calls give them a better look at you in a number of different ways. The audio quality is clearer than phones provide. They can check that you're able to dress properly and groom yourself. They can see your body language, and you can see theirs.

On the whole, video calls provide a much better experience and clearer communication between both parties. If both parties have computers with webcams and microphones, why *wouldn't* they want to use them for something as important as a job interview?