Answer:
This is to ensure survival of desired bacterial cells.
Explanation:
The growing of transformed bacteria with ampicillin-resistant gene, ( in presence of ampicillin ) is for the ampicillin to disrupts the the process of synthesising protein which inhibit its chemotherapeutic actions, therefore promoting the survival of transformed bacteria.
However, bacteria which are without plasmid, which are not transformed,without the ampicillin resistance gene, (since the action of ampicillin is not block,) will not be able to grow in the ampicillin medium, and therefore will be killed.Thus aiding the survival of transformed bacteria only.