onLoad & onUnload are mainly utilized for popups that appear while the user enters or leaves the page. Another significant use is in combination along with cookies which should be set upon arrival or while leaving your pages.
For instance, you might have a popup asking the user to enter his name on his first arrival to your page. Then the name is stored in a cookie. Furthermore, while the visitor leaves your page a cookie stores the current date. Next time the visitor arrives at your page, this will have another popup saying something like: "Welcome jiya, this page has not been updated since your last visit 8 days ago".
Another common utilization of the onUnload and onLoad events is in making annoying pages that instantly open several other windows as soon as you enter the page.