Because you've already got the directory open, but might not know how you got there. Especially if your directories are a cluttered mess, like mine are, and you

Because you've already got the directory open, but might not know how you got there. Especially if your directories are a cluttered mess, like mine are, and you found the directory using the file search feature. And the "save" dialog is not completely standardized, e.g., not all programs start you out at the same place, and some force you to navigate all the way from root.