With Adult Language Learners I like to use textbooks because they give our conversations a skeleton. Phrases, contextual vocabulary, correct spelling and accepted grammar are the bones in a conversation's body. I fail to remember every word or turn of phrase and therefore I rely on these manuals to jog my memory and keep me in standard French protocol for your benefit.
With only weekly lessons the visual and aural review of what we have discussed as found in a textbook is useful for building the new necessary pathways in the brain for acceptance of the new language. Whereas the experienced authors of these texts teach hundreds of new language learners; their exercises stretch brains around the new patterns and sounds of a foreign language. It would take me much more time outside of lesson to prepare new material for an upcoming conversation, as well as time during our lesson to ensure I have presented the correct meaning of a word or phrase as we get into higher level learning.
Infants and toddlers hear language for thousands of hours before ever really producing it. It is full of errors, then, through school years of practice and discipline, it matures. By using textbooks we can abbreviate the time it takes before production because our eyes can follow while we hear and the paper will remember what was learned for us. They allow us to analysis and chew on new material through repetition outside of a lesson.
If we had day after day and hour after hour of French immersion, we could do without a textbook. When I was learning Japanese and Italian as an adult I used the text to be my memory (I was busy with my life while learning these languages) because I could not devote myself to an immersion program for my only task to accomplish.
Your time with me is costing you some cash and I want to present you with the tools to halve or quarter this cost by giving you a review tool: our textbook.