Why in JavaScript can we omit params in first argument (callback) of reduce?

We have this code:

const nums = [1, 2, 3]

const sumNumbers = (sum, num) => sum + num

// WORKS!
let a = nums.reduce(sumNumbers, 0)

// ERROR!
let b = nums.reduce(sumNumbers(sum, num), 0)

Why when I set (sum, num) in callback sumNumbers I got error:

ReferenceError: sum is not defined

There is a name for this way of programming?