The TikZ manual has this little gem of wisdom in its description of the
\foreach command (emphasis mine):
In the easiest case, ‹variables› is a single TeX command like
\point. (If you want to have some fun, you can also use active characters. If you do not know what active characters are, you are blessed.)
The LaTeX wikibook has a few paragraphs about them, and they appear to be a perculiar quirk of Plain TeX, but outside of the example given, I can't think of any sane reason to use them. While I'd consider myself somewhat proficient with LaTeX, I'm a novice when it comes to Plain TeX and understanding how the whole thing actually works.
What is the purpose of active characters, and why would you use them? I'm curious as to why the TikZ manual authors may have written that statement.