The Lettuce Leaf is simply a large barrier in-between Borgers as they are made. Borgers are wrapped in Lettuce Leaves in groups of up to 5 for preparation to be served to customers. Lettuce Leaves are also very nutritious, which is why they come with Borgers. Other then that, they're not a hard barrier to pass at all.
Sometimes however, when they are served, the leaf falls off the Borger and both are thrown away due to ruined quality. The leaves also come in purple and blue flavors, making them even more exciting to wrap a Borger in. Finally, the surface area of a Leaf changes depending on the amount of wrapped Borgers, usually 2, and is very flexible in size depending on the breed of lettuce.