In "A Worn Path" by Eudora Welty, why does Phoenix Jackson talk to herself?

Phoenix
Jackson in byhas a mission---to travel the worn path and get the medicine that her grandson
desperately needs. She has been making this trek for three years since he swallowed lye and
burned his throat.

This journey is not easy for Phoenix.  The author
describes her as quite old.  Phoenix does not know her age. She carries an umbrella that she
uses as a cane to ward off any critters that try to attack her.  Her poor vision often deceives
her; for example, she sees a scarecrow that she at first thinks is a man and later a
ghost.

Her attitude toward life amazes the reader.  She humorously jokes
about the difficulty of the path and the things that she encounters. 

Her
banter serves two purposes. It tells the reader about Phoenix and her beautiful personality. 
What she says provides much of the humor to give some lightheartedness to a serious story. 
Phoenix may have always talked to herself to keep company.  Much of what she says is addressed
to the outside forces that might prevent her from accomplishing her goal.

It
is interesting that Phoenix has learned how to respond to the prejudice of the white people she
encounters. When the hunter calls her granny or implies that she is going to town to meet Santa
Claus, Phoenix does not argue or try to set him right.  She keeps her mouth shut and goes on her
way. Of course, she is elated that he dropped a nickel that she now has in her pocket.

One of the obvious aspects of Phoenix comes from her memory failure.  Everything works
as good as possible for Phoenix except that she has spells of drifting off in her
thinking:

A big black dog came up out of the weeds by the ditch. She was
meditating, and not ready, and when he came at her she only hit him a little with her cane. Over
she went in the ditch, like a little puff of milkweed.

Down there, her senses drifted away. A dream visited her, and she
reached her hand up, but nothing reached down and gave her a pull. So she lay there and
presently went to talking. 'Old woman,' she said to herself, 'that black dog come up
out of the weeds to stall you off, and now there he sitting on his fine tail, smiling at
you.'

Because of her age, senility, dementia,
or strokes are possible reasons for her forgetfulness and her talking to herself.  When she
drifts off or does not remember what she is doing, it is obvious that Phoenixs body may outlive
her mind.