And a minimal pair:
- Πες μου ό,τι θέλεις. "Tell me whatever you want."
- Πες μου ότι θέλεις. "Tell me that you want."
The comma is part of the word, a bit like apostrophes in English words such as "o'clock", and is regarded as a separate symbol from the punctuation mark comma -- the mark in ό,τι is an υποδιαστολή hypodiastole while the punctuation mark that separates clauses etc. is a κόμμα comma. (A bit like how in English, an apostrophe is considered a different mark from a closing single quotation mark, though they usually look the same.)
The reason why the orthography is different is, as jaye16 said, to distinguish between ό,τι, whatever (the indefinite interrogative), and ότι, "that" (the conjunction).
In ancient Greek this distinction was made with a space, viz., ὅ τι / ὅτι. So the comma is a modern orthographical development, nothing more.
“I'll cook whatever you like“ is correct, but “I will cook whatever you like” is not. As “I’ll” is a contraction of “I will”, are these sentences both right or both wrong?
(1) if the υποδιαστολή is formally different from the κόμμα as the apostrophe from the closing single quotation mark, ¿is there a separated key on the keyboard for it?
Also (2) I keep receiving a typo message of an extra-space when I write «ό,τι», and I think that it might be because I'm writing it with the key for the κόμμα in my keyboard, and maybe there's a different key for the υποδιαστολή...
if the υποδιαστολή is formally different from the κόμμα as the apostrophe from the closing single quotation mark, ¿is there a separated key on the keyboard for it?
Just like English keyboards don't have separate keys for "hyphen" and "minus sign" or for "period" and "decimal point".