Everyone has a different learning style..some people are auditory (learn by hearing), some are visual(learn by seeing) and some are kinaesthetic (learn by doing). When a person teaches others he will use the same method to teach as the method by which he learns. So a trainer who is a visual learner himself will tend to teach by using visual displays.
Now, during a training session, it is very useful to understand the learning style of the trainees so that the trainer can cater to them accordingly. A good way to understand a particular trainee's learning style is to ask a few questions at the beginning of the class, when each trainee introduces himself. Asking a trainee about his hobbies is a good way to get insight into his learning style.