why do we need <strong> tag because it displays the same effect as<B>tag

Answer 1

They may be look visually the same, but they have different meaning. <b> stands for "bold" and is visual effect. But strong makes information in it IMPORTANT. Screen readers, bots, search engines etc. refer "strong"-ed texts differently.

http://www.w3schools.com/tags/tag_strong.asp

Answer 2

usually they appear the same. <b> looks like <strong> as <em> looks like <i>. using <strong> sounds like mature coder you know.