What is the purpose of a liner on a cookie tray?

The purpose of a liner on a cookie tray primarily serves to prevent sticking and for easy cleanup. When baking cookies, using a liner, such as parchment paper or a silicone mat, creates a barrier between the cookies and the surface of the tray. This prevents the cookies from adhering to the tray, which can cause them to break apart or lose their desired shape when removed. Additionally, liners make post-baking cleanup much simpler since they catch any drips or crumbs, allowing the tray to be cleaned easily without excessive scrubbing.

The other options, while they might touch on practices in baking, do not accurately capture the main function of a liner. For instance, decorating cookies can be done with toppings or icing, but liners do not serve that purpose. Providing extra flavor is not a function of liners, as they are neutral and do not impart any taste to the cookies. Similarly, maintaining temperature is related to the baking process and not something a liner directly contributes to. Therefore, the correct choice reflects the practical benefits liners bring to the baking process.