You might never use it. As far as I know, you should never use it in a fiction book. One problem is that it slightly disrupts reading. Of course, I complain about that for a lot of things.

There also seems to be no reason to use it. The reader is going to read everything in the book. And "pay attention to me" seems to be something the author would say, so in a way it breaks the fourth wall. I can't think of any way of speaking that corresponds to bold, and all-caps would seem to handle everything that could be in bold.

Non-ficction is different. I use it often in my examples, to draw attention to the part the reader needs to pay attention to. (Some authors use italics for this purpose and then note after the quote that they have added italics for emphasis. I cannot do that for every example.) On the topic of connecting independent clauses, I considered two choice:

I went on a diet, and so I lost 20 pounds.
I went on a diet, so I lost 20 pounds.

There is – potentially – another interesting usage. Suppose I am in some discussion forum, and I post a message. I might expect, quite reasonably, that people will see my message but not read it. Or they might start reading it and lose interest. If my message is not useful to them (or not useful enough to be worth reading), then the system is working well. No one thinks my messages are always important for everyone to read, not even me.

But, in that situation, I will sometimes use bold. The idea is to put in bold something short and snappy, and something that might tell the reader to read more if my message might actually be useful to the reader.

However, my usage does not seem common, so you are on your own if you use it now.