Astral using her new body, wanted to immediately go on, and experience more human things, but first she needed clothes. She could have sent her spectres to do the job for her, but she wanted the whole human experience.
And using the poor psycho woman's pick pocketing skills to get by, would be the start of her human, criminal life. She wanted to steal from someone use that money to buy actual clothes at mall, probably get some human food.
Astral had used soul extraction, then used her skinwalkers trait, to steal or take over the body, by matching her soul wavelength with the original owner of the body, tricking the body into believing she was still the same woman.
Each soul has a connection with the body, that goes beyond normal means, that's why possessions from spirits work, it's because they aren't replacing the original soul, they are actually blending with it, but also taking control behind the scenes.