DEI is to prevent the kind of favouritism/nepotism that prevails in a lot of society (e.g. "the old boys club"). A suitable example would be the recent hires by the U.S. administration - people are being given high ranking jobs just because they're loyal/friendly to a certain person and nothing at all to do with their competency to do the particular job.

The purpose of DEI is to allow the most qualified person to get the job despite the overt racism and sexism in society.