Contrast on a page draws our eyes to it; our eyes like contrast. If you are putting two elements on the page that are not the same (such as two typefaces or two line widths), they cannot be similar - for contrast to be effective, the two elements must be very different.
Contrast is kind of like matching wall paint when you need to spot paint - you can't sort of match the color; either you match it exactly or you repaint the entire wall. As my grandfather, an avid horseshoe player, always said, "'Almost' only counts in horseshoes and hand grenades."
The basic purpose: Contrast has two purposes, and they're inextricable from each other.
One purpose is to create an interest on the page - if a page is interesting to look at, it is more likely to be read.
The other is to aid in the organization of the information. A reader should be able to instantly understand the way the information is organized, the logical flow from one item to another.
The contrasting elements should never serve to confuse the reader or to create a focus that is not supposed to be a focus.
How to get it Add contrast through your typeface choices, line thicknesses, colors, shapes, sizes, space, etc. It is easy to find ways to add contrast, and it's probably the most fun and satisfying way to add visual interest. The important thing is to be strong.
What to avoid Don't be a wimp. If you're going to contrast, do it with strength. Avoid contrasting a sort-of-heavy line with a sort-of-heavier line. Avoid contrasting brown text with black headlines. Avoid using two or more typefaces that are similar. If the items are not exactly the same, make them different!