[fasquoika]

This is how both TeX and m4 do quoting by default so presumably common when they were created (mid-eighties and early seventies respectively IIRC)

[kergonath]

It’s an ASCII approximation of the proper typography for single quotes (except when you’d use inverted quotes for some reason, not sure how common it is in English).

Similarly, LaTeX uses `` and ‘’ for double quotes because the opening and closing symbols are not the same in properly typeset texts.