You need it to stay safe and anonymous while accessing them. These parts of the internet can get you into serious trouble, especially if you're accessing illegal websites there. Not to mention that you can even find child pornography, which some countries punish without hesitation.
All in all, the benefits of staying anonymous while browsing the Darknet are numerous (even if you're using the 'visible' internet). Avoiding government surveillance, avoiding ISP throttling, and bypassing geo-restrictions are only some of them.