Without a doubt it is the most used button on Windows, and if it is not used as much on Mac it is due to the fact that most people use the multi-column view, which is super useful when navigating through many folders.
But if we ever have another view, we may be interested in Parent Folder. Instead of giving 'backwards', we can go up to the higher level, something that is often lacking and that we do not have available in the Finder As they point out to me in the comments, we can do with the keyboard (cmd + above) but not with the mouse.
The download is completely free and the application weighs very little, so by trying it you do not lose anything. In Snow Leopard it does not look good, but if you go to the source of the post you can download an icon that adapts.
Source | maximum
Download | Parent Folder
Command + cursor up
plus cursor up command also does this.