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.
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.
Standard: lol no
Modern: BG/x, UR/x, Burn, Merfolk, Zoo, Storm
Legacy: Shardless BUG, Delver (BUG, RUG, Grixis), Landstill, Depths Combo, Merfolk
Vintage: Dark Times, BUG Fish, Merfolk
EDH: Teysa, Orzhov Scion / Krenko, Mob Boss / Stonebrow, Krosan Hero
Standard: lol no
Modern: BG/x, UR/x, Burn, Merfolk, Zoo, Storm
Legacy: Shardless BUG, Delver (BUG, RUG, Grixis), Landstill, Depths Combo, Merfolk
Vintage: Dark Times, BUG Fish, Merfolk
EDH: Teysa, Orzhov Scion / Krenko, Mob Boss / Stonebrow, Krosan Hero