What is perfectly clear to you must be made clear in minutes to people unfamiliar with your work. Your slides can help you to accomplish this goal. Since words have limitations you may want to illustrate what you cannot verbalise, what you want to emphasise, or what takes too long to describe. Graphs, drawings and photos can often explain what language cannot. Use your slides to hold attention, enliven, clarify, restate, explain, and interpret.
Ears have trouble accepting numbers and abstractions.
In addition, quantities and relationships must be visually compared. When you digress from the topic of the slide on the screen, use a blank slide, coloured and shaded, not white, to darken the screen. Furthermore, it will confuse your audience if you say one thing orally and you display something else visually on the screen. An outline of your major topics should be your second slide, just after your title slide. Pie charts are good for showing relative parts of the whole.