Why do I almost never have to clear my browser for major websites, but do on others?

On many websites in my company and other smaller sites I am often told to clear my browser cache, and indeed that fixes problems. I assume this is because the browser cache has become corrupt.

However, I rarely have to do this on very popular web sites - iCloud, Quora, CNN, etc.

So what is the difference? What are they doing, or not doing, that accounts for this difference?