Legally, you can only vote once. But if you vote early, you can enable more than one vote to be cast.
Voting in election day is an application of queueing theory. When voter turnout is high, as it is expected to be this year, the queues can become long. Sometimes very long. The lines in Ohio in the 2004 election are infamous. As a result, many voters balked* or reneged** before casting their votes. Alexander S. Belenky and Richard C. Larson ask: Did election queues decide the 2000 and 2004 U.S. presidential elections? Their analysis is summarized in their ORMS Today article.
Practically speaking, queue lines can be reduced one of three ways:
- Fewer voters enter the queue
- There are more voting booths and people processing voters
- Ballots are shorter
Voting early or voting absentee shortens the queues on election day by addressing issue #1. So while you can cast only one vote, casting your vote early means that you can keep the queues shorter on election day and possibly enable someone else to vote who otherwise would not be able to. This is meaningful in practice, since many voters cannot wait in line because of family responsibilities or shift work. So far, 2018 seems to be setting records for early voting. I voted absentee because I will be in Pheonix for the INFORMS Annual Meeting on Election Day.
* Balking: The voter decides not to enter the waiting line.
** Reneging: The voter enters the line but decides to leave before voting.
Related posts:
Punk Rock Operations Research 
November 1st, 2018 at 12:09 pm
I can think of a few other ways to shorten queues at polling places: allow straight ticket voting via a single check box (I’m not a fan, but it does speed things up); get useful information (as distinguished from campaign noise) out ahead of time and encourage people to make informed choices before queuing up (good luck with that); and write ballot proposals in clear English, so that unprepared voters don’t have to try to parse them while hogging a booth.