Changing a string identifier does not magically add semantics. The only difference between the two sets of tags is the "semantic" ones make your pages a bit longer. They are, to all intents and purposes, equivalent.
From a purely visual perspective, yes that's true, those tags are essentially equivalent. However, from a semantic perspective, that is certainly NOT true.
For example, assume that you are "listening" to the page rather than viewing it. What does <b> mean? It means "bold". Ok, so what does that mean semantically when that section gets read? Absolutely nothing. "Bold" describes presentation, not content. On the other hand, if it was marked up with <strong>, that has semantic meaning, which can be interpreted to be audibly different than the surrounding text. The same holds true with regards to <i> and <em>. <i> meaning "italicized" has no semantic value, but <em> has a semantic meaning of "emphasis".