IMPORTANT UPDATE: A revision to this article has been made in light of the Magic 2014 rules update. Please refer to the correction below for the current ruling.

At the beginning of each sanctioned match you are to present your deck for shuffling by your opponent. At this time you should also be presenting your sideboard. In a constructed match, your opponent is allowed to count your sideboard facedown to make sure that you do have a 15 card sideboard (As of the release of Magic 2014, a player’s sideboard may be between 0 and 15 cards, rather than the old 0 OR 15; as such, while your opponent is still entitled to count your sideboard face-down to make sure the number of cards you’ve got in your sideboard matches the number on your decklist, it is okay for you to have fewer than 15 cards!) In a limited match, as there is no set number that your sideboard can be, your opponent cannot count your sideboard.

One reason you are required to present your sideboard is so your opponent knows where you will be getting cards from when sideboarding. The other main reason has to do with constructed sideboards, so your opponent knows if your sideboard is the legal and correct size.