What is the sense of the is.single() function in R?

I stumbled on the function is.single() and do not understand what is does. The description says:

is.single reports an error. There are no single precision values in R.

And indeed, if I try to test different things inside the function I get an error. Example:

is.single(1)
Error in is.single(1) : type "single" unimplemented in R

I see no sense in a function that seemingly always gives an error. What is the function for?