Tag for yourself
Tagging in del.icio.us offers you the chance to organize bookmarks so that they make sense to you. It's more efficient than the folders that are offered by browsers because you can tag one single bookmark with several "labels" and find that bookmark through any of those tags.
Tag for others
Many people will tag bookmarks in del.icio.us so that not only they, but other del.icio.us users will be able to find them.
Use others' tags