A decorative element is not essential to your wiki's skin, but adds some value to it. This value could give your wiki a certain attitude or a distinctive element over a competitor's wiki, or help your visitors perform some task on your wiki, such as finding the information they are looking for using the search box feature.
It is important to use the decorative elements in your MediaWiki skin sparingly, so as not to detract from the wiki's content, which is why most of your visitors are on your wiki in the first place.
Having said that, a white page with black text is not very welcoming for your visitors, and decorative elements can help make your wiki more visitor-friendly.
As a general guide, it is probably OK to use decorative elements when:
Your page is a somewhat sparse
Something on your page needs more focus than it currently has
The wrong elements on your page have too much focus (for example, through poor choice in layout), and you need...