"The Mother" is a poem about abortion in which the narrator laments her children "never made." While nurturing children, including feeding, is a central facet of motherhood, here the food imagery focuses on the feeding that is never done.  

In the first stanza, the narrator uses the second person "you," gaining distance with that pronoun while addressing both herself and presumably other mothers who have aborted children for whatever reason. She speaks of the "sweet" (the candy or sugary treat) that she never had the chance to give them. In the last line of the stanza, she reverses the imagery, recognizing that children also repay the nurture of the mother with their presence. She mourns she will never "return for a snack of them, with gobbling mother eye." This refers to the way mothers often fall in love with their children and see them as the most adorable beings in the world.

In the second stanza, the narrator moves to the more personal first person voice and imagines feeding her unborn children: "I have eased/ my dim dears [imagined, ghostly children] at the breasts they could never suck." 

The food imagery conveys a sense of loss and sadness at the simple, basic part of motherhood—physical nurturing—that has never been.

I see food imagery in only one line of Gwendolyn Brooks' poem "The Mother." That's line 10: The speaker is full of regret that she will never "Return for a snack of them, with gobbling mother-eye."

This particular use of food imagery is a little surprising. We usually think of a mother's love being expressed through giving food to children, not through gobbling up the children. But then again, we having sayings in English to express affection toward children -- such as "You're so cute, I could just gobble you right up." -- that this line is probably drawing on.

I don't know if it's ever possible to know why a writer does one thing and not another in a poem. What we can know, of course, is how we as readers make sense of the poem and how the poem, to us, works or doesn't work. I'm sorry not to be able to answer that part of your question.