Some users are fine with using their registrar's domain servers, others prefer a dedicated DNS provider to escape the constant barrage of upselling (looking at you Register.com and GoDaddy), or to get access to advanced features (read: native IPv6, anycast, geographic targeting, that sort of thing. DNSimple has a RESTful API and an iPhone app, for example), or - mostly - to get better support than registrars are typically known for providing. (Go ahead and try to find someone at Enom who wants to talk to you about updating glue records. I'll wait.)
^^This is sort of like saying, "I don't get services like Dropbox. Rsync works fine for me." If you don't think you need an UltraDNS/Dynect/DNSimple type of service, then you probably don't, and there's nothing wrong with that.
The monthly $8 plan is for DNS service. The other fees listed are for yearly domain registrations. The language could certainly be more clear around that.
I don't get services like DNSimple. Why would I pay monthly just be able to pay to register domains? What am I missing?