This tag is used to highlight the important words. The strong tag makes the text bold.
<p> You will be given <strong>only 1 Hour<strong> to complete the work. </p>
You will be given only 1 hour to complete the work.
Note: Although Em tag makes the text italic and Strong tag makes the text bold, these tags should not be used for formatting the text.
The reason we use sementic tags is because they give the meaning to the webpage and not just the structure. In case of <b> and <i>, they just tell the browser how the text should look. Whereas in case of semantic tags, they give the additional information to the browser.
Semantic tags mainly comes into use when the visually impaired or blind people use the screen reading softwares. While reading out the matter to the blind people, the software will read it as the html authour wantes it to be read.
The author tells the browser which words to emphasize, which words to give more importance, which are the quotes and all the rest of it by using the semantic tags.