I have written this post with pre-service trainee teachers in mind... Naturally, a key consideration for you during your training is likely to be one of securing a job (and a salary) by the end of it. However, as one might expect, many head teachers are reluctant to recruit new staff without first seeing them teach.

And rightly so. Recruitment itself takes time and money. And the stakes are high for school leaders for whom the quality of teaching is usually a prime concern.
Also, teaching is a complex, dynamic profession. It requires intelligent, flexible, responsive individuals to make effective use of a range of strategies for ensuring that the pupils in their classes make progress. By observing you teach a lesson, and witnessing your practical abilities, a potential employer is in a much stronger position to confidently make you a job offer.