Once you are registered you can vote in person or via absentee. If you don't exist, it's probably better to do it by mail where the chances of being caught are smaller.
Also, if you are registering people unlikely to vote, you can 'help' them fill out their absentee ballot
There's one other way such registrations can be helpful. If one assume one party registers a lot of fraudulent voters, one would also assume they are registered with that party's affiliation (for example, DEMOCRAT). That drives up the dem portion of registered voters in that precinct.
If the eventual election is challenged in that district, lawyers will use the proportional party affiliation in their legal arguments.