Blocking is used in Much Ado About Nothing. Why has the director blocked the play in the way he/she has?

This is a difficult question to answer, since your
question implies that you are asking about one director in particular, yet you don't
make reference to any particular production.  Each director of any play will make very
specific choices about sets, costumes, and even the characters actions and behaviours
(blocking) based upon their interpretation of a
play.

Plays, by the very nature of the sort of work of
literature that they are, are inherently incomplete, and must be staged in order to
become the "complete" story.  Blocking, or the staging of the actor's movements and
placing on the stage, is a key aspect of what is missing in the script that is added in
rehearsals for a particular production by a director.  Yet, it is not possible to answer
you question completely without reference either to specific scenes that are of interest
to you, or mention of a particular production about which you would like a general
critique of the direction.

An example in Much
Ado of a scene that varies in terms of potential blocking choices made by the
director is actually a scene that is not in the script at all -- the deception of
Claudio and Don Pedro at Hero's window.  Don John tells these two in Act III, scene ii
that if they will come with him, he will show them proof, by their own eyes, that Hero
is unfaithful.  Don John is planning to have his henchman Borachio lure Hero's waiting
woman, Margaret onto the balcony and woo her under the name of Hero, while the other
three observe from below.

This scene is not included in the
play that Shakespeare wrote, but is often staged by directors so that the audience might
see, along with Claudio, the ruse that Don John has cooked up.  Directors will use the
staging of this invented scene to either make Claudio more or less gullible when it
comes to his accusations, based upon how believable the wooing of Margaret (as Hero)
appears to be.

This is one example of where a director's
blocking makes a statement about the how the audience is intended to experience the
behaviour of the characters, but certainly there are others, as there are in any
play.