Why must we register to vote?

Why must we register to vote?

Is it for the integrity and trustworthiness of the vote results, or something else?

Here are some opinions on the subject:

1)      https://answers.yahoo.com/question/index?qid=20080224085017AAdnBeA

2)      http://usgovinfo.about.com/od/thepoliticalsystem/a/voterreg.htm

3)      http://www.usa.gov/Citizen/Topics/Voting/Register.shtml

There are many other links on this subject, too.

Of course if you want change, then vote, and even go through the effort to vote.