The Temporary Internet Files folder contains a kind of travel record of the items you have seen, heard, or downloaded from the Web, including images, sounds, Web pages, even cookies.
Storingfiles in your cache makes browsing the Web faster because it takes your computer less time to display a Web page when it can call up some of the page's graphics or even the entire page from your local Temporary Internet Files folder.
All those files stored in your cache take up space, so from time to time, you may want to clear out the files stored in your cache to free up some space. I've had some friends & family who never knew about this & had never cleared their cache. The end result was a VERY slow computer over time & they didn't know if they had a virus or if their hardware just needed to be upgraded or replaced. Turned out that one click of "Clear Cache" (which took a very LONG time by then) made their computer run much faster than it had in a long time, saving them both time AND money (turns out they didn't need to replace their PC after all).
Now I'm not entirely sure how this works on an iPhone, but since there's a place to to in the settings to clear your cache, I'm guessing that it's probably as important to do here as it is on your desktop or laptop PC.
Tap on the "Settings" icon on your iPhone. Scroll down to the 3rd section, 4th icon down (Safari) & tap on that. Now scroll all the way to the bottom & right above "Developer" you'll see "Clear Cache". Tap on that & it will ask you if you're sure you want to clear the cache. Tap on the top button to go ahead & clear it, or the bottom one to cancel.