Answer
The reason for studying Arabidopsis is that the features which make it an attractive experimental system (simple genetics, small size, rapid lifecycle) allow us to learn many general things about the basics of plant developmental genetics. These general lessons can then be applied to other specific systems such as cassava and plantain. Arabidopsis, at least in the ideal, should save time in learning critical things about important food crops.
Work Step by Step
Review the chapter on Arabidopsis and see why it is a preferred experimental system. Then use you imagination to see why this gives us general knowledge about plants quickly. This general knowledge, as in the case of our understanding of floral developmental genetics (see the last pages of the chapter), can then be applied to other plants.