This is an answer, I wrote in response to a recent blog post by Erica Sadun:
Many people ignore a particular use of the minimap - understanding the complexity of some portion of the code, without having to look at each line. In most languages, complexity increases with the level of nesting of code. This is usually difficult to estimate when reading the code itself, since the brain is focused on what the code does, rather than how nested it is. By obscuring the text, and leaving only a bird’s-eye view of the layout, the minimap might help pinpoint parts of the code, needing improvement. It can be very helpful when working with someone else code, or with a code base, which you haven’t touched in a while.