What is the purpose of a phone interview before in-person?

It's just a simple screening process as to whether you match the standards of a company's interview standards. Also giving you the basic overview of the job is thier goal.

basic qualifications.  not to waste time of in person interviews.  especially if salary isn't what your expectations are.  more often than not, an HR recruiter will tell me what the salary or wages are up front so we're on the same page.  if they couldn't budge or negotiate, we wouldn't move forward to the next steps and waste time.  

Sometimes there are very basic things that are deal-breakers in which case there would be no reason to waste everyone's time with an in-person interview.

For example:

Not willing to work Saturdays?   Deal-breaker.

Have terrible phone skills?  Deal-breaker.

It's just a primary interview to determine if you fit the basic requirement.  Then a second in person interview gives more of an impression of how you handle yourself, how you dress and act, and more information about your problem solving and thinking on your feet.  

They don't want to waste face time on people that are not qualified.

To weed out the lesser candidates.