The cache should be cleared regularly for several reasons. Depending on your settings, the cache can grow quite big and use a lot of disk space on your computer. And that's even if you don't visit some pages ever again. The more information that is saved in the cache, the slower your computer will be browsing the web. Deleting the cache data helps to troubleshoot, helps to increase the loading time of web pages and increases the performance of your computer.

If the browser doesn't load the new version of the site, even if there have been changes on the site since the last visit, the cache can cause issues with the view. The website might look like it has not been fully loaded or is wrongly formatted. Maybe you already wondered why a website is showing outdated contents, despite it having been updated. That's probably because of the cache. Sometimes data, which is stored in the cache prohibits loading of up-to-date contents. When you delete the stored cache data, the new version can be retrieved. In many website related issues, the cache is the root of all evil. That's why your web developer/web designer or IT-Support first asks you to clear the cache before investigating further.

By the way: since the cache stores website data, you can tell from the saved information which sites had been visited in the past. Regarding privacy and data security clearing the cache could be also interesting for you.