Here "or" is used as a real alternative conjecture, and therefore without any point. In the following examples, the "or" joins equivalent expressions:
England, or the nation of shopkeepers, would never be asked to join such an alliance.
We perceive, or are conscious of, nothing but changes, or events.
As a reason for the insertion of the comma in these two examples, it may be said that the repetition of an idea already expressed does for a moment stop the flow of the sentence. A real alternative, on the other hand, forms an essential part of it, and is within its current.
XVII. In cases where no point would be used before a conjunction, a comma is inserted if the conjunction be omitted.
I pay this tribute to the memory of that noble, reverend, learned, excellent person.
In the following examples no point occurs; for it cannot be said that a conjunction is omitted. To insert the conjunction would be to express a slightly different shade of meaning:
A grand old man.
Three tall young soldiers.
"Old man" is virtually a single word and in fact many languages use only a single word to express the idea.