Why is this an issue?

Unused parameters are misleading. Whatever the values passed to such parameters, the behavior will be the same.

Noncompliant code example

function doSomething(a, b) { // "a" is unused
  return compute(b);
}

Compliant solution

function doSomething(b) {
  return compute(b);
}

or

function doSomething(_a, b) {
  return compute(b);
}

Exceptions

When arguments is used in the function body, no parameter is reported as unused.

function doSomething(a, b, c) {
  compute(arguments);
}

Also, the rule ignores all parameters whose name starts with an underscore (_). This is a common practice to acknowledge the fact that some parameter is unused (e.g. in TypeScript compiler).

function doSomething(_a, b) {
  return compute(b);
}