It's true not all horses need constant supplementation. Some supplements are given to horses the way we eat vitamins and any excess gets eliminated through natural processes. Once in awhile you may need to focus on repair or pain maintenance if something comes up that is an acute situation. You should ask your veterinarian if you are not sure what you should do. Quality products improve performance and horse health much faster.
You can target an issue and know you are not adding to the problem by using supplements that contain fillers and bulk that are strictly for profit by some manufacturers. I have people ask why can I get this cheaper at the bulk food store. Well, first off it's not the same product and second you are paying for fillers that make it look like you are getting the same amount but quality and quantity are very different. You know the old adage "You get what you pay for" !