Why are fairies included in the plot of A Midsummer Night's Dream?
The fairies’ magic, which is central to the fantastic atmosphere of the A Midsummer Night's Dream, symbolizes the supernatural power of love (e.g. the love potion), and contributes to create a surreal world. Even if the misuse of magic eventually causes chaos, magic ultimately solves the love tangle at the centre of the play.
In this sense, it is important to consider the fact that, while Shakespeare uses significant aspects of the Elizabethan belief in fairies in his play, he also alters the conception of fairies. In fact, Shakespeare transforms the whole conception of "fairy" from wicked tricksters to harmless "shadows.", a conception which was perpetuated by later authors:
The fact that we now see fairies as tiny, harmless creatures with wings and magical powers that live in the woods is due to this play. Although Shakespeare gives prominence to the Elizabethan folk belief of fairies by highlighting them in the play, he changes the popular idea of fairies in A Midsummer Night's Dream from wicked spirits to shadows and dreams, a transformation which lasts to this day (see source below).