OPK tests confusion

So I keep seeing you guys keep your opks tests through out the month to track kinda think. And there all dated. Can someone explain to me why? If the reading is no longer accurate after set time *and I know mine change with time) why keep them to track? Wouldn't it be inaccurate anyway? I'd love someone to explain cause we've been trying for years and mayne I should be doing it that way...