In theory:
The point is to deregister people who have either moved out of the area or have died. There’s not a good system in place to automatically update the system for this, especially in the case of people who move.
So some jurisdictions adopt a position that if you haven’t voted in your district for a “long” time then that’s reasonable evidence you have probably moved house and are voting elsewhere, which isn’t all that unreasonable, except sometimes the “long time” can mean people who sit out an election cycle for some reason or another are at risk of having their names removed, and they’ll find out when they try to vote and either be unable to vote or have to jump through a bunch of hoops.
Some of the controversy is that in practice some of these purges are done in ways and at times that make it look like the intent is less about conforming the registration to current residents and more about purging voters that are statistically less likely to support the people doing the purging, with many of the voters only finding out at the polling station.
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