This site for example: http://mosaicscience.com/story/how-bee-sting-saved-my-life-poison-medicine
When I get a piece of snail mail that says "Important! Open Immediately" on the outside, I always think "Important to whom?" - it is rarely important me.
- remember your language/country,
- store a temporary authentication token (when logging into a website, checking the "Remember me" checkbox) so you don't need to re-login every day,
- remember your name after you filled a commenting form on a blog post so the next time you comment it's already there
- etc.
Bad cookies are used to track you across different websites so they can target advertisements to what you have searched on Google or what pages you have visited.
If you are concerned about your privacy you should use an extension such as Ghostery to block trackers and other bad cookies and then, as trebor says, accept all the other ones coming from 1st party sites since they usually want to help you
The problem is that the web isn't stateful, as we term it, so each request you make is a totally blank slate—except for cookies. If they do any expensive computations and then cache them in the session (temporary data associated with your browser through a cookie) clearing your cookies will force them to do it again.
In my experience it's better to trust 1st-party cookies that the site you're visiting directly may set, rather than trusting all 3rd party cookies set by folks like Google or Facebook.
A simple example is that you indicated some preference. The only way for the browser and server to retain that preference is via a cookie.
Websites that use cookies in any way must prompt the user about it.