Learners have different strengths and a wide range of approaches will make a book useful to more readers. According to Howard Gardner's theory of multiple intelligences people have different strengths and will therefore benefit from different approaches. In that vein, teachers commonly try to use varied teaching styles to cater to different students' abilities.
Some people are very good at picking up the sounds of words, while others are more apt at memorising the way they are written. For some the patterns are easily recognisable while for others actually writing them will better help them commit them to memory. These are just examples of strengths that will require different approaches in how the content is presented. A combination of audio (even with video), text, explanations and lists for rote learning can help learners with different abilities.
While it is difficult enough to come up with a textbook taking only one learning style into account, it is useful to keep in mind that what works for oneself isn't necessarily going to be the most useful method of study for the learner.