Hypodiastole

Because ancient Greek was written without spaces.

Scribes copied text in one continuous run of capital letters, a style called scriptio continua. Usually readers sorted out the words themselves, but some letter strings could be cut two ways. A small hook between the letters, the hypodiastole, told the reader: break here. Its opposite, the enotikon, a little joining stroke, said the reverse: these pieces belong together.

Modern editors of papyri and manuscripts use ⸒ when they want to reproduce exactly what the scribe wrote. In some fonts it is drawn as a low comma, in others as a bold half circle like a reversed C.

Is the comma in ό,τι a hypodiastole?

In function yes, but you type a normal comma.

Modern Greek still has one word that uses the old idea: ό,τι (“whatever”) is written with a comma to tell it apart from ότι (“that”). Greek grammars call that comma a hypodiastole (υποδιαστολή). Standard practice, including Unicode's own advice, is to type the ordinary comma there, not ⸒. Using ⸒ in everyday Greek text can break search and spellcheck.