Nobody is saying that clearing your caches DIDN'T free up some space for you. What they're saying is that this is not an effective way to increase your phones speed or performance. The cache is there to hold commonly used files and data that apps need. It helps apps open and run faster. If you delete it all, the apps have to figure things out from scratch, and it takes longer. Kinda like when you wipe all data to flash a ROM, the first time the phone reboots it takes forever. And, once you run these apps again, they will write their data into the cache again, so it will start filling up right away anyway. Yes deleting the cache will technically clear some internal space, but it's not going to give you any performance. Apps will actually run slower if they don't have things in the cache.
Like on your computer when you go online, webpages are cached and then the next time you visit them, some of the data is already remembered by your computer, and they will load faster for you. If you delete all your internet files and caches and start visiting some of your usual websites, they would load slower because none of the data is in the cache already.