What are some roles and functions of the fairies in Shakespeare's A Midsummer Night's Dream?

The fairies, both individually and collectively, serve a
number of different roles and functions in Shakespeare’s A Midsummer Night’s
Dream, all of them important.  Among the significant impacts of the fairies
on the play are the following:

• Puck, the fairy
  who first appears, adds an invaluable dimension of wit, energy, cleverness, fun, and
  mischievousness to the work.  Puck is one of the prime sources of the play’s comedy. He
  is also one of the most memorable creations in all of comic
  literature.


• The fairies as a whole add to the magic and
  mystery of the play, often making it seem dreamlike, as the title of the play would
  suggest.


• The fairies as a whole seem to be spirits of
  vitality and liveliness, moods wholly appropriate to a
  comedy.


• The conflict between Oberon and Titania adds to
  the suspense of the play and even gives it, at times, a slightly darker edge.  Just as
  the main plot often involves conflicts between males and females, so does this element
  of the plot involving the fairies.


• Oberon and Puck are
  the two great tricksters of the play, thus contributing to the work’s comic
  intrigue.


• Although Puck possesses supernatural powers (he
  can circle the globe in less than an hour), he is also capable of making mistakes that
  lead to various comic complications, as when he initially places love potion in the eyes
  of the wrong lover.


• The presence of the fairies gives
  Shakespeare the opportunity to enhance the charm of the play, as when Titania is sung to
  sleep by other fairies.  Yet the presence of the fairies also gives Shakespeare the
  opportunity to invent further, highly comic complications, as when a deluded Titania
  courts a highly appreciative Bottom.


• The presence of the
  ever-obliging fairies also gives Bottom an excuse to demonstrate his comical love of
  luxury and pampering, particularly from Peaseblossom, Cobweb, Moth, and
  Mustardseed.


• As these names suggest, even the monikers of
  the fairies are often comic. Shakespeare manages to create fairies who seem mythical and
  mysterious but who also seem wholly appropriate to an English
  setting.


• The reconciliation between Oberon and Titania is
  perfectly appropriate to a play culminating in happy
  marriages.


• The concluding moments of the play, in which
  the fairies bless the unions of the various couples, provides the play with an
  extraordinarily harmonious ending. Shakespeare in this work reveals his ability to
  depict supernatural elements in writing that is almost literally magical. The opening
  lines of Puck's final speech give some flavor of the beauty of the play's phrasing,
  especially in the language spoken by the
  fairies:

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PUCK:
If we shadows have
offended,

Think but this, and
all is mended,
That you have but slumber'd
here
While these visions did
appear.