Yes, both are indispensable to the program! Neither one can be used successfully without the other.
The Teacher's Guides contain the actual lessons and instructions, as well as the directions for each activity in the workbook. They are the core of the curriculum, guiding you through what you teach for each part and explaining all the concepts. They also include a wealth of teacher resources and guidance on how to teach Essentials successfully.
The Student Workbooks contain the student response portions for hundreds of exercises, as well as sample words for certain concepts explained in the Teacher's Guides, game boards, and practice activities.
You can see samples from the Essentials Teacher's Guides and Workbooks here.