This is a very basic question that I have that makes me think that I do not fully understand the basics of why we add urea to the gels. I have tried to look in the literature but I did not find any consistent answer. 

The point is that when you read any protocol to make an electrophoresis gel with urea they always suggest to pre-run the gel without your samples to "remove" the urea. My question is related with this specific thing...if you remove the urea before adding your samples then why it make sense to use it? Is it any physical process related with the polymerization of the gel that only occurs in the pressence of urea?

I hope any of you have an explanation. Thank you in advance!!