What is the point of a side deck in Yu-Gi-Oh?

Answer 1

In Match Duels (basically, you're playing best out of three with another duelist) a Side Deck can be used. A Side Deck is entirely separate from your main deck of cards and can only hold a max of 15 cards.

After a duel is completed, before the next duel starts each player can look at the cards from their Side Deck and swap them with cards in their Main Deck. Cards included in the Side Deck would be ones to counter and adjust your strategy now that you have seen your opponent's deck in action. You might want to include extra Spell/ Trap removal against this one strategy or remove a card from your deck that doesn't function well against your opponent. In any case, cards taken out of the Side Deck must be replaced with cards from your Main Deck and vice-versa.

Once players are done, the second duel begins. If there is a need to go into the third a final duel, each player can exchange cards in their Side Decks again if wanted.

Answer 2

A side deck is a small pool of up to 15 cards, that you can switch into your deck in between the duels of a match. You have to switch one card out for each you put in, and must revert the deck back to how it was originally before the next match.

It is useful because you can put counters to various deck types in there, instead of trying to include them all in your original main deck and possibly slowing it down or making it harder to do its own thing. As an example, take Dimensional Fissure. It might not do anything for your original deck, so there's little reason to use it. However if it is in your side deck, then if you play a deck where it is very useful against, you can side it in after duel 1.

Answer 3

The Extra Deck is the new name for the Fusion Deck. Since the introduction of Synchro Monsters, the term Fusion Deck was no longer strictly accurate. So it was given a new name.

A side deck is 0-15 cards that in between games in a match (3 games in a row) you can switch those cards for ones in your Main Deck.

If you use either or both of these, you must have them there with you on the table, before the duel starts. (Check the official rulebooks that come in all starter decks for more information.)