Macbeth refers to himself as a yellow autumn leaf. How is the imagery suitable to the outcome of the play?

In this particularly poignant speech, Macbeth is referring
to his way of life.  He comparies his life to a withered yellow leaf. And indeed, things
have not worked out as he planned.  He is fully aware that Lady Macbeth is a victim of a
"mind diseased"  and that all their crimes have brought
sorrow--

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perilous stuff

Which
weighs upon the heart.

The
threat of Malcolm and Macduff joining forces and marching on Dunsinane is fast becoming
a reality, and Macbeth sees no true means of escape.  In this desperate hour, Macbeth
evaluates his life and knows that he has achieved
nothing:

And
that which should accompany old age,

As honor, love,
obedience, troops of friends,

I must not look to have .
.,

Approaching what seems to
be the end of his life--"the yellow sear," he feels as if he has lived long enough.  His
life has withered instead of flourished, and like a dead leaf he can only wait for the
final winds--the English forces-- to blow it from the tree into
oblivion.