Designers make lots of layers and lots of layer copies during the course of a design, and they often “forget” to name the layers appropriately. We understand though, they’re busy people, right? In the coding world we deal with similar issues. When programmers don’t effectively comment their code, anyone coming in after them has no way of knowing why certain choices were made, or perhaps even what a particular block of code should be doing. Likewise, a single designer may know—as the creator of the file—that Layer 12 copy is the little red left-pointing arrow after the link text in the second-row third-column of the page layout, but if anyone else picks up this file, they won’t know what that layer is without taking several annoyingly time-consuming steps to figure it out.
It is crucial that designers name their layers appropriately, so they are understandable at a glance.