Around 98% of all Internet Users accept cookies. Those who do not for sure run into problems with some important sites.
Mostly you can watch but not participate (like: a personalized Google does not work without cookies).
Affiliate networks track your commissions with cookies.
E-Commerce sites track your shopping cart with cookies.
They are everywhere and for sure needed. On the other hand: you are trackable, if you do not delete them once in a while. I recommend a "clean up" day once a month to erase all cookies and history from your browser.
There also anonymizer suites like steganos is offering...