Why cookie sheets have sides?

* Preventing spills: The sides help to contain batter, cookie dough, or other food items, preventing them from spilling over the edge and making a mess.

* Easier handling: The sides provide a lip or rim that makes it easier to grip and lift the sheet, especially when it's hot or full of food.

* More even baking: The sides help to distribute heat more evenly by creating an enclosed space, leading to more consistent baking results.

* Enhanced functionality: Cookie sheets with sides can be used for more than just baking cookies. They can also be used to roast vegetables, bake bread, and even as a makeshift serving tray.

However, not all cookie sheets have sides. Some are completely flat. These are typically used for:

* Thin cookies: Flat sheets are better for baking thin cookies like biscotti or crackers, as they allow for even heat distribution and prevent over-baking.

* Toasting: Flat sheets are also ideal for toasting bread, nuts, or other ingredients, as they expose the food to direct heat.

Ultimately, the choice between a cookie sheet with sides and a flat one depends on your individual needs and preferences.