Why should Kira stay alive?

Are you referring to the way in which Kira's society casts off those with deformities or illness. If so, she was kept alive because of the love of her mother. It was the Council's opinion that those born with deformities were useless and unproductive in society.

So she could do the robe because nobody else could do like her and she kinda knows how to make colors and she's important.