Suppose you've got a roman numeral class. If you treat an instance as a number it works as a number ($fourty_two = $rn_six * $rn_seven;); however when you print it out it represents itself as the roman numeral form (print "The answer is $fourty_two\n"). To get the string version you might explicitly use $as_roman = "$roman_instance" to force stringification (granted in most cases you'd probably be either interpolating it with something else or concatenating it with another string). Or another plausible occurrence might be some operation that just returns the string value, i.e. return "$roman_instance".

Again, it's a very contrived case, but there are occasions where you will use $a = "$b".