the country of Genosha used them to keep a mutant workforce for a while, but that was before Magneto kicked out all the bad guys there. then the AI that made the Sentinels went nutso and we destroyed that too. recently it was uhhh a super advanced Sentinel from the future that came back to our time and infected a technopath to create a cyborg that wanted to make a 'utopia' where mutants were enslaved and used for labor, again. like the original Genosha but worldwide.

so yeah! Sentinels suck.