What is the dramatic importance of Macbeth's decision to go to the witches in Macbeth?

I assume you are refering to the end of Act III scene 4,
after Macbeth has organised for the murder of Banquo and has seen his ghost dramatically
during a banquet in front of his Lords. At this point, he tells his wife that he intends
to visit the witches again. He himself interprets the importance of this action for us,
so pay particular attention as to how he explains the reason for wanting to make this
visit:

I will
tomorrow

(And betimes I will) to the Weird
Sisters:

More shall they speak; for now I am bent to
know,

By the worst means, the worst. For mine own
good,

All causes shall give way: I am in
blood

Stepp'd so far, that, should I wade no
more,

Returning were as tedious as go
o'er.

Macbeth thus thinks
that he has done so many bad things now that he might as well carry on. We have seen his
gradual progression from being cajoled to committing murder to cold-heartedly organising
the murder of a former friend himself, and now he is, as he says, "bent to know... the
worst." He has given himself over to evil, and recognises that the witches are the
representatives of evil on his plane, and thus seeks to know the rest of how the story
will turn out.