"strong" is an intention and allows other things to define what style(s) that intention should be represented with.
It's not often that you want to represent something as a "bold" style without representing it as having strong emphasis, but there are times. A header is a good example of a place where you might want the header to be styled bold, but strongly emphasize a single word in the header. Since <header> doesn't apply the styles that - say - h1 does, it would be perfectly reasonable to script your header as
``` <header><b>The <strong>Worst</strong> Case Scenario</b></header> ```
... especially if you didn't want all of your headers to have a bold style applied.
Obviously there are a few other edge cases but I will admit that the vast majority of the times would naturally call for a strong tag, over a bold tag. And for those majority cases, I'm really glad that it's "strong" rather than bold, because "strong" is meaningful in different ways than "bold" and it always makes more sense, to me, reading it back as html telling me how I should interpret the word, rather than just how it should look.