Header tags are the visual headlines and titles that you see on your site. They are vital as a visual sign of importance, but, in addition, Google uses them as such. Of course, you can create this effect by enlarging the paragraph font and changing its color, but that should be avoided. You should really use the correct HTML tags for this effect, which are the HTML tags <h1>, <h2>, etc., until <h6>. If your CSS is correct, the tags will be styled appropriately, usually with a visual hierarchy, in which the <h1> is the largest and the rest are progressively smaller. Besides being visually recognizable, they can be used also to create structure and hierarchy in your texts. In addition, I advise you not to use more than the first three levels, as it makes the structure harder to understand, though, of course, there can be situations for which you may have to disregard this.