The role of the fairies in A Midsummer Night's Dream is to create both the conflict and the resolution in the play. They do this by manipulating the human beings in the play while also remaining separate and distinct from the human beings.
Puck creates conflict in the play when he accidentally mistakes Lysander for Demetrius and enchants Lysander instead. Oberon asks Puck to fetch him a flower Oberon has seen Cupid hit with an arrow. The flower now serves as a potent love potion, and if the juice of the flower is placed on any being's eyes, that being awakens to fall in love with the first creature he/she sees. Oberon wishes to use the flower to manipulate his wife into giving him the foundling. While waiting for Puck, Oberon witnesses Demetrius being cruel to Helena in the woods, and when Puck returns, Oberon asks Puck to use the flower on Demetrius, whom Oberon says Puck will recognize by his "Athenian garments" (II.ii.269). Puck has no idea that there are actually two Athenian couples in the woods that night and accidentally enchants Lysander to fall in love with Helena instead then later enchants Demetrius to fall in love with her as well.
The fact that both men are now pursuing Helena instead of Hermia creates a great deal of conflict among the characters. They get into a great many arguments. Helena accuses Demetrius and Lysander of scorning her and also accuses Hermia of being in on the joke, as we see in Helena's line, "Lo, she [Hermia] is one this confederacy!" (III.ii.195). However, Puck makes amends by properly uniting Demetrius with Helena, and once again, uniting Lysander with Hermia. Hence, we see that the fairies play the role of creating both the play's conflict and the resolution through the use of their magic.
However, while they interact with the human world by manipulating it, critic George A. Bonnard points out that the role of the fairy world is actually to remain separate and distinct from the human world. The separation shows us that while the human world, the world of reality, can be manipulated by the fantasy world, the world of fantasy actually has nothing to do with reality, even though fantasy may sometimes look like reality. We especially see reality look like fantasy when the lovers become enchanted. However, at the end of the night, the lovers awaken half believing that what they experienced was a dream and half believing it was real. The fairies make them believe it was a dream because they wish to remain separate and distinct from the world of reality. Hence, another one of the fairies' roles is to portray Shakespeare's theme of fantasy vs. reality and to show us that reality is often manipulated by fantasy.
Fairies in a Midsummer Night's Dream are tiny troublemakers who at times can cause trouble for humans. Puck, a rogue fairy, creates havoc for Bottom, the hapless full-of-himself actor and creates a vicious love square between Hermia, Lysander, Helena, and Demetrius. Shakespeare uses these fairies as comic relief and also uses them to bring about certain actions in the human world. Without the tampering of the fairies, the characters would not have had the happy ending they did.