Back in the Model T days of internet browsers, there was a browser that got confused when the cache got too big, or old or something. That slowed things down (by slowing down, I mean that the browser became unusably slow). The answer was to dump the cache and the browser would return to normal. Unfortunately, a lot of "tips and tricks" sites still think that is valid advice. I think it was in the last millenium that all the major browsers figured out how to maintain the cache properly.

There are some unusual circumstances that might benefit from a larger cache. Possibly viewing large video files multiple times might qualify. You won't be doing this over dialup.

I find that the sites I visit regularly have code built into the web pages that force the browser to ignore the cache and download the web page directly from the server. Eg, if the pages on Newagtalk didn't have the code built in, you would be frustrated that you weren't seeing the latest messages. So, having a larger cache would be of little benefit for web sites such as Newagtalk that constantly change.

Now, why have a cache? With slow internet connections (especially dialup), you can go back to view the same page multiple times without waiting for the page to come down again. I don't  personally see any benefit as I have a fast connection and most sites I look at are constantly changing anyway. 

Long and short, don't bother messing with the settings. They don't magically improve your internet browsing. If you have a specific problem accessing the internet, or one site in particular, you can clear the cache as a troubleshooting step.