Why use `concat` instead of `push` in this instance?

Push()

The push() method is a generic method similar to call() or apply(). It will mutate your array (or object), pushing a new value into it.

Push: MDN

Concat()

The concat() method returns a new array, with the values merged. This also avoids mutation side effects.

Concat: MDN

If some other code has a reference to the existing array in actionQueue, using concat will not affect that.

var a1 = [1];
var b1 = a1
a1 = a1.concat([2])

var a2 = [1];
var b2 = a2
a2.push(2)

console.log('b1', b1)
console.log('b2', b2)