In ruby using a single quote is faster than double quotes as we can see in the message : Is there a performance gain in using single quotes vs double quotes in ruby?.
So why does everyone use double quotes (or both)???
Some examples :
(I know, it's not everyone but the majority.)
EDIT 1 : Interpolation is not a reason!
Double quotes are not always with interpolation or anything special like this, in Sinatra :
it "defines HEAD request handlers with HEAD" do