I think, if I remember correctly, that b and i are used to only alter the style of text, while strong and em are used for emphasis in something like a paragraph of text.
So you could use b in something like UI breadcrumbs, but of you wanted to strongly highlight something as an author of some text, you'd use strong.
I'm not really sure we need these though. As for my breadcrumbs example, I think everyone would use CSS rather than a b element.
So, I kinda understand the idea, but I've never needed it.
A thing that maybe comes to mind is that inner monologue in books uses italics sometimes. So using i would make sense more than em. But when wanting to emphasize/stress a word or phrase an em would make more sense than i.
So you could use b in something like UI breadcrumbs, but of you wanted to strongly highlight something as an author of some text, you'd use strong.
I'm not really sure we need these though. As for my breadcrumbs example, I think everyone would use CSS rather than a b element.
So, I kinda understand the idea, but I've never needed it.
A thing that maybe comes to mind is that inner monologue in books uses italics sometimes. So using i would make sense more than em. But when wanting to emphasize/stress a word or phrase an em would make more sense than i.
+1 regarding <spoiler>!