FHSS and DSSS are two methods utilizing spread spectrum technology. The idea behind spread spectrum is to spread the signal across a wide range of frequencies. This makes spread spectrum hard to detect as it often looks like noise. It also makes it less susceptible to interference and jamming. Although spread spectrum, by definition, is an inefficient use of spectrum, it results in a much more reliable transmission.
In Figure 2.1, we see that narrow band interference would cause a great deal of degradation to a narrow band signal. However, it would cause very little interference with a spread spectrum signal. In essence, this is a design decision that emphasizes reliability over efficiency.