What’s the point of _curry1() in ramda.js?

I am reading right now the source code of rambda.js and don’t understand the point of _curry1().

function _curry1(fn) {
  return function f1(a) {
    if (arguments.length === 0 || _isPlaceholder(a)) {
      return f1;
    } else {
      return fn.apply(this, arguments);
    }
  };
}

Functions that are curried with this function can be called without arguments.

const inc = (n) => n + 1
const curriedInc = _curry1(inc)

// now it can be called like this
curriedInc(1)   // => 2
curriedInc()(1) // => 2

// but it works only with no arguments provided
const add = (a, b) => a + b
const curriedAdd = _curry1(add)

curriedAdd(1)     // => NaN
curriedAdd(1)(1)  // => TypeError: curriedAdd(...) is not a function
curriedAdd()      // => function f1(a)

Question:
It seems to me that you can’t chain arguments with this function at all. What are use cases for this function? How different is curriedInc()(1) from inc(1)?