Of course, you can run a blog without these legal documents. But a blog is a business, and a business needs to be legally protected.
The purpose of this document is primarily to provide you and your audience with clarity as regards their rights, your rights and whether any liability rests on you, your readers, and any third parties.
The main things that this document will protect you for are your liability in respect of complaints or lawsuits. For example, if you give advice on your blog then you must protect yourself by clearly specifying whether this information is or is not intended to be professional advice.
It also prohibits illegal or unlawful activity on your site, as well as the posting of material that may be harmful, malicious or defamatory. It also prevents visitors from gleaning commercial gain from any of the material on your blog. If you are selling goods or merchandise through your blog then a refunds, returns and cancellation policy will be included.