Your browser keeps local copies of the images, scripts and styles a site sends, so the next visit loads instantly. That cache is a genuine performance win — until a site changes and your browser keeps using yesterday's files alongside today's.
Cookies are different. They are small notes a site leaves so it can recognise you, keep you signed in, and remember settings. Clearing them signs you out but rarely breaks anything permanently.
That is why 'clear the cache' is such reliable advice for one broken site: it removes mismatched files without touching your accounts, provided you clear cached files rather than everything.