The C language gave different meanings to single quotes and double quotes:

'c' // a byte
"c" // array of bytes

Decades later, a difference persisted but, in most languages, changed its character. Single and double quotes now often change how string interpolation works:

"#{foo}" # interpolate the string value of variable foo
'#{foo}' # literal, no interpolation

As far as I know, this comes from Bourne shell. In the weird world of shell scripts, it actually turns out to be useful, but in the more structured world of most programming languages, it has approximately zero real-world use cases.

Suppose that you do literally want to write “#{” in a string:

'#{foo}' # intentional, but looks like a mistake

Python got this right. It used to be right in JavaScript.

Single quotes should be interchangeable with double quotes. Interchangeability allows you to use the easier-to-type single quotes most of the time, but switch to double quotes for easy single-quote escaping.