Can someone explain to me the purpose of a sideboard? Is it necessary to have one?
The purpose of a sideboard is to be able to adapt your deck to overcome ideally any challenge.
Id say it's pretty necessary for competitive play, the purpose of the side board is to have additional cards that can change your deck to suit the meta game. For instance let's say your opponent is running a reanimator deck, putting cards like {tormods crypt} or {grafdiggers cage} in your main deck may not make sense, because not everyone plays reanimator, so you play your first game, and realize you have great cards in your deck that are worthless while paying against reanimator, you can then swap those out and replace them with cards that hinder reanimator from working. This is just one example, there are literally thousands. But as I said, for competitive play, sideboards are necessary.
If you use a sideboard, it has to be exactly 15 cards. No more, no less.
Also you cannot have more than 4 of any card in your deck an sideboard COMBINED. This means that if you have 4 {Ratchet Bomb} in your main deck, ou may not have any in your sideboard, and vise-versa.