What are some uses for a wiki such as SoyWiki, that is only accessed locally? Of course, this doesn't just have to apply to SoyWiki, it could apply to any type of wiki that can only be accessed on a user's local machine.
Wikis are much better than plain text files or paper at organizing large amounts of information.
This is useful especially when you're undertaking a large project that requires you to keep track of lots of things, like a software project or a book. A wiki is like a bunch of index cards, in that you can shuffle things around and organize information into different clusters. Except that a wiki lets you navigate and search your notes in much more powerful ways than index cards and paper notes.
I also think of a wiki as a four-dimensional version of a paper notebook. You can not only write down and read back your notes, but you can also jump, worm-hole style, from any page to any other page through hyperlinks, and you can also bend time and see a snapshot of what your notes looked like any point in time in the past.
Here are some links that might give you some more ideas:
Wiki (assuming feature set: versioning, search, labels/categories) is pretty useful documentation system, even for one user.
Wondering when you entered something? Or trying to find something? Or suddenly remember that you need old version for some reason (for example old password you already edited away or something).