I've been going over Advanced Grammar in Use and came across Unit 9 Rule E I'll cite a part of it:
However, we use "will", not "be going to", when the main clause refers to offers, requests, promises, ability, etc.:
There are two examples the first of which I'll omit.
If you look to your left, you will see the lake.
= you will be able to see, you are going to see suggests: "I know this is what you can see when you look to your left"
The question is not about the rule itself but rather about the explanation about the difference between the usage of will/be going to here. I don't quite understand what they mean by saying "I know this is what you see.." Is it just about letting them know that you have already seen that or what?
I would like to ask to clarify this "be going to usage" and tell me a real life situation where this might be useful, in other words another example, thanks!