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.
> 
> Why is it some employers want to do video calls, such as Skype, for
>  interviews instead of just a phone call?
> 
> 
> 

Body language.

You've probably noticed that you have more information conveyed to you when you talk to someone in person, rather than on the phone.

The look on their face, the tilt of their head, the shaking or nodding their head - these all indicate what the person is thinking as much as their words do.

This is what employers are trying to gain (at least to some extent) by using video rather than just phone calls.