Why is this an issue?

Having all branches of a switch or if chain with the same implementation indicates a problem.

In the following code:

if b == 0 {  // Noncompliant
  doOneMoreThing()
} else {
  doOneMoreThing()
}

switch i {  // Noncompliant
  case 1:
    doSomething()
  case 2:
    doSomething()
  case 3:
    doSomething()
  default:
    doSomething()
}

Either there is a copy-paste error that needs fixing or an unnecessary switch or if chain that needs removing.

Exceptions

This rule does not apply to if chains without else, nor to switch without a default clause.

if b == 0 {    //no issue, this could have been done on purpose to make the code more readable
  doSomething()
} else if b == 1 {
  doSomething()
}