Parasites are often isolated in their host, and are often sessile. Both of those would make finding a mate difficult or impossible. Being a hermaphrodite allows sexual reproduction and its beneficial effects in such very small, unmoving populations.
Work Step by Step
Look at some of the examples of parasites in the book and look for common traits (here, population size and motility), and try to imagine how these would affect reproduction. How might having both male and female sexual organs overcome any problems resulting from that.