Why is “ss” equal to the German sharp-s character 'ß'?

Question

Coming from this question I\'m wondering why ä and ae are different(which makes sense) but ß and ss are treated as equal.

Answer 1

If you look at the Ä page, you'll see that not always Ä is a replacement for Æ (or ae), and it is still used in various languages.

The letter ß instead:

While the letter "ß" has been used in other languages, it is now only used in German. However, it is not used in Switzerland, Liechtenstein or Namibia.[1] German speakers in Germany, Austria, Belgium,[2] Denmark,[3] Luxembourg[4] and South Tyrol, Italy[5] follow the standard rules for ß.

So the ß is used in a single language, with a single rule (ß == ss), while the Ä is used in multiple languages with multiple rules.

Note that, considering that case folding is:

Case folding is primarily used for caseless comparison of text, such as identifiers in a computer program, rather than actual text transformation

The official Unicode 7.0 Case Folding Properties tells us that

00DF; F; 0073 0073; # LATIN SMALL LETTER SHARP S

where 00DF is ß and 0073 is s, so ß can be considered, for caseless comparison, as ss.