MTG Salvation

A great deck building technique which is not talked about very often is the "Elephant" technique. That is a method where you conceive of the best deck build for your particular deck for each matchup and then whittle it down to a full 75. A quick Google search brings up this article from 2013, and I'm sure there are others: http://www.starcitygames.com/article/26317_The-Elephant-Method-A-Case-Study.html.

If you use this method you will often end up with multiple 1-of cards in your SB, because your MD is reconfigured on a card by card basis per matchup.

In addition to what the above posters said, it is often done to make card by card upgrades to your main deck for a specific matchup. Let's say that you have 1 card in your main deck that is truly horrible in the matchup you're currently facing. If you have 1 card in your sideboard that is great in that matchup, that is a swap which you will always want to make. True, you may not draw it. But if you do, it's much better than if you drew the card that it replaced.