The key point here is that this escape clause is used to wrap expressions that you would only use once.
It may be easier to see if we put it in a different context.

Imagine you are at someone's house and are ready to leave. So you go around and say goodbye. But then, for some reason, you can't leave just yet. When you get ready to leave, again, a half hour later, it is a little awkward to go around and say goodbye to everyone again.
If you suspected that might happen, you might say, "If I don't see you again, I guess this is goodbye." But then when you come back later to really say goodbye, it is not so awkward since you didn't actually do it before. You just put in a proxy that turns into your goodbye if you don't come back. But then you do come back to say goodbye and the previous escape clause is invoke and cancels the previous statement.

This comes into play in any case where you might only want to say something once with regard to an event. For example, "Hope your game goes well this weekend." Whereas you wouldn't use it for phrases that are normally repeated at the end of a conversation, e.g. "Have a good day," or "Talk to you later."

The key point here is that this escape clause is used to wrap expressions that you would only use once.
It may be easier to see if we put it in a different context.

Imagine you are at someone's house and are ready to leave. So you go around and say goodbye. But then, for some reason, you can't leave just yet. When you get ready to leave, again, a half hour later, it is a little awkward to go around and say goodbye to everyone again.
If you suspected that might happen, you might say, "If I don't see you again, I guess this is goodbye." But then when you come back later to really say goodbye, it is not so awkward since you didn't actually do it before. You just put in a proxy that turns into your goodbye if you don't come back. But then you do come back to say goodbye and the previous escape clause is invoked and cancels the previous statement.

This comes into play in any case where you might only want to say something once with regard to an event. For example, "Hope your game goes well this weekend." Whereas you wouldn't use it for phrases that are normally repeated at the end of a conversation, e.g. "Have a good day," or "Talk to you later."