If you break down the action in smaller steps, you can see the answer.
Example: Single clicking selects an item and double clicking opens/executes the item. But, when you double click, you are essentially also performing the single click and hence, you see the item being selected first before opening it.
In tap and hold, you are tapping the item which is an action on it's own and the holding is an action on top of it.
It provides a nice way of undoing your contextual menu without having to cancel it and perform the primary task i.e. tap to open. If there was no 'open' in the contextual menu and someone thought they were doing the click but happened to 'click' it for a bit longer end up with the contextual menu (I have seen this happen many times with non adept mobile users), this provides them to still 'open' the item rather than being forced to 'open in new tab'