The banners module is one of DNN's old time classic modules. Both the module and the Vendors directory have seem some changes lately, but the central functionality is the same - you can display banners on a page which update when the page is refreshed or moved away from and then back to.
It often seems that the module takes license to display banners as it likes, but this is not the case - chose carefully the order in which you add your banners as this determines the order in which they appear in the module. Updating modules does not change the order - so add them in the right order the first time. By chosing the right sequence you can achieve interesting result - for example I have recently used the banners module inside the ZDModuleWrapper to enable 3x3 different colored banners to always appear on the page at a different time. That is there is always 3 banners of a different color showing at one time and never two of the same color. Once the website is launched, I'll add a link to my blog so you can see the results.