In order for an applicant to be invited for interview, the screeners believe that that applicant has the potential to be a good addition to the upcoming class. At our school, each application is screened by at least two screeners. In making this decision, the highlights of their application (including demographic data, breadth and depth of experiences, etc) are considered, within limitations of what can be considered during the screening process anyways. This is why it's important to ensure the highlights of an application are not buried in the midst of paragraphs. Help us, help you.
The adcom ultimately may not be in agreement with the screeners, but by then, the interview may have already occurred and preliminary scores compiled. The percentage of applicants who 'essentially waste money going to interview' is very low from what I've seen. Schools also have incentive to avoid wasting our resources. Screeners who offer too many interviews 'inappropriately' are of course spoken with as perhaps we may have missed something, or perhaps some additional instruction is needed on what qualities are looked for. Just my thoughts