For the computer, single quote (') and double quotes (") are different text characters.
There are situations where you are expected to type double quotes (") and typing a single quote (') won't be understood by a program. For example, when the search operator for exact phrase search in a search engine like Google requires double quotes, it typically won't work with a single quote.
It's also not possible to type two single quotes ('') to "make" double quotes. For the computer, that's two separate single quote characters, not a single double quotes character.
In ASCII encoding, the bits for the single quote and double quotes characters are:
| Character | Bits |
|---|---|
' | 0100010 |
" | 0100111 |