I have seen in many posts where the keywords that they are targetting are in bold or underlined or italicized? does this help for SEO for better ranking?
Yes it is important. You can bold important things in your post so that readers can notice important things...
YES it helps on-page SEO. But also don't forget to use proper internal linking, to relationate your resources - within your pages - by targeting anchor texts... it helps too.
Well, I would not think it as any important factor if i look from the SEO point of view but I would say from the visitor perspective I would consider it as using the bold and Italic font will help in improving the visibility among the readers and the probability of getting the text read is much more higher, because as far a the human mentality is concerned people are more likely to read the content that is much more visible to you. So, from SEO perspective using the bold and italic font is just not going to make any difference but it will hamper effect on the visitor point of view.