Employment history and graduate admission Why do some graduate schools require an employment history for their application forms?  Additionally, is it likely that they verify said history via one's social security number?
Why do some graduate schools want to have employment history on
  graduate school application forms?


I can think of a few things:


If you're not coming directly from undergrad, they want to know what work you've been doing.  In some cases, it may be entirely irrelevant, but nonetheless, they ask.  While references are typically obtained from academic institutions, if you've been in industry for a while, one of your recommendations may come from your employer...
If you are coming directly from undergrad, you may still have been employed during school or have completed some number of internships.  Again, while they may not be relevant, there's no reason not to ask, especially if they can positively affect your application.
Either way, employment, depending on what you did, could demonstrate responsibility, leadership, potentially research ability, etc.



  Is it likely that they verify it with SSN?


This almost seems like you're trying to determine if they can catch you in a lie--they do not need a SSN to do that.  Don't fabricate anything.