My open tabs are kinda like my working memory. I could close them and reopen them as needed, but it is easier to just click a tab then to try to find the thing again. In addition, sometimes the pages I am looking at have dynamic navigation, and closing and reopening the page will require a good deal of navigation to find the section I was looking at. Plus, the overall time to reload the content, where if I just have the tab open there is no download time.
Also, the tab icons help me remember what I was looking at. I will often keep tabs open for what I wanted to read later.