The main reason to block cookies is to prevent tracking cookies.
With tracking cookies enabled you get things like:
- Search for a birthday present. Wife logs in on the same computer and gets lots of targeted adds for that or very similar items.
- You Google for a specific item and buy it. The next months you get an overdose of adds for an item which you already actively looked for and already bought.
- You simply do not like others to track your every move from website to website.
Ofc, there is quite a difference between tracking cookies and so called functional cookies (e.g. cookies to remember preferences set for a webpage, one of the very things why cookies where created). However these days cookies simply get abused and the default setting should be to disabled them except for a very small number of white-listed sites.