The young mothers you see talking to their infant children are doing exactly what they should do.

Talking to a baby has been shown to produce many effects, including stimulation of brain development and emotional bonding between parent and child…

As early as 4 to 6 months, children begin to babble, repeat sounds, and learn that they can get attention through vocal expression. This is the beginning of expressive language.

At 6 to 8 months, a child begins to develop receptive language, recognizing his or her name and common words such as mama, dada, bye-bye, bottle, or binky.

Source: post-gazette.com
http://www.post-gazette.com/pg/06158/696102-114.stm