what exactly is the purpose of a sketchbook and how do I meet the criteria?

Although having good quality artwork in your sketchbook is important, its primary purpose is to support your final pieces. Therefore at least 50% of the page should be writing. The sketchbook is there to explain to the examiner your reasoning behind your final artwork. This includes any inspiration form other artists or daily activities whether in terms of themes, technique or colour scheme. The pages should also include the journey which led you to your final piece including previous attempts and insight on the evolution of your artwork. You should then move on to analysing your own work, setting out what you like about it and what you could improve on. You should also precise why you did what you did: why you used certain colours, mediums, techniques etc. Finally, to hit top marks, you could suggest how you can push your study of this area further and set out potential linked future projects.
Should your textbook only include writing, you will be able to achieve decent but overall average marks. To hit the higher marks you will also need to show some flair and effort in regards to presentation and make sure you have a few high quality pieces included in the sketchbook to support your writing.