[80386]

Fraktur ß is a ligature of s and z; the current form came into use in Latin-script German because the Latin script already had ß, for the ligature of s and s.

Some sort of orthographic device is needed - s between two vowels means the first vowel is long and the consonant is voiced, and ss between two vowels means the first vowel is short and the consonant is voiceless. So it's useful to have a different case for when the first vowel is long and the consonant is voiceless:

Busen /bu:zən/

Busse /busə/

Buße /buːsə/

Vowel length isn't predictable from spelling in cases of consonant clusters and other digraphs: Hand /hant/ vs. Mond /moːnt/, Bruch /brux/ vs. Buch /buːx/, etc. So ss could've been used for both - or sz, although there are some words spelled with sz as a sequence of s and z, like Szene /stseːnə/.

[leipert]

Not really _needed_. Swiss orthography doesn’t use _ß_