Once a teacher has a solid understanding of the methodology and philosophy involved in various kinds of Language Teaching approaches, he/she can begin to design lesson plans orientated at facilitating this process in the classroom.
While adaptability and spontaneity are essential personality traits in a teacher, without thoughtful lesson planning a teacher is basically teaching "at random" without a logical and pre-determined course to follow. Lessons conducted with poor (or no) effective planning will result in students struggling to conceptually understand the relevance and usefulness of the language they are learning. They may end up practicing language forms they do not instinctively understand, and without effective prompts to use it in a "productive" way, it is likely to go in one ear and out the other. Entrenched errors and a lack of "conceptual understanding" can develop easily as a result of this.
The amount of planning and thought a teacher puts into his/her lessons really depends on how much they want the class to achieve. Using pre-selected textbooks helps to select and organize language items, but a textbook can be limited when it comes to facilitating:
-> Recycling and Revision of Language
-> Combining Language Items
-> Combining Phonics/Pronunciation issues with Key Language
-> Identifying and Rectifying areas of particular difficulty for YOUR students
-> Identifying and Rectifying areas in need of Remedial work
-> Realistic Engaging/Presentation and Production/Activation of Language