I don’t like cookies. You never know what is really being collected or what is being done with the information.
In saying that, I use WordPress and MyBB as site software and I understand they use them to remember the logged in users and commenters details so they don’t have to enter them again and for no other purpose.
I can assure you that I don’t use cookies for any purpose outside the WordPress and MyBB functionality. Feel free to block them on any site if you don’t feel comfortable with them. It won’t cause any issues beyond you having to retype your details in to leave comments or repeated log in.