Is it only for screen reader software? because browser renders both type of tags semantic and presentational in same manner.
For example:
for browser for us and for css <strong> and <b> is same. what is the purpose to semantic tag over presentational tag.
is it for screen readers only or it's for better management of code?
if it's for developer strong and b both can produce same result on browser.
<strong> and <b> is a poor example in this case, as they are just historically used in a wrong notion. A lot of people started marking everything bold with <strong>, thus destroying the original intent. My phone browser does not make <strong> bold for instance (although the standards suggests making it bolder on screen).
The idea behind semantic tags is to provide some description about the content. So <strong> tag for menu items does not make any sense, while it makes sense to use it to mark part of a sentence as if it was pronounced louder.
With HTML5 semantic tags make a lot more sense, because the content part of the page is clearly outlined, and every tag inside the content is a usable meta data. Search engines are already good at this, but everyone else isn't.
I suggest reading about the whole concept of Semantic Web.
Semantic markup allows scripts to understand context. This may be beneficial for screen reader software, but it will also be beneficial for Google and other search bots.
According to HTML specs, <strong> and <em> communicate emphasis, whereas <b> and <i> simply mean "display bold" and "display italic". <b> and <i> should be used in instances where emphasis is specifically not desired. For example, when italicizing a book title.
When search bots are trying to gain semantic understanding of content, it is reasonable to assume they give greater preference to semantic tags.
In theory, an audio page reader could read <strong> text in a different, slower, more emphasized tone of voice. It wouldn't do that for just <b> because that's only a typographical hint for graphical presentation. A terminal-based browser could use underlining to reproduce <strong> as an alternative to bold if it's not an available effect on the terminal, where is principle it wouldn't make sense to do that for typographical <b>. A search engine could give more importance to <strong>ed words.
In practice, I don't think any of these examples actually happen — partly, as HeavyWave says, because decades of poor-quality markup have erased any difference between them that could usefully be drawn — but it demonstrates the philosophical difference.