You don’t have to apply to become a trainee driving instructor, but as mentioned, the benefits of a trainee driving instructor licence is that it allows you to learn and prepare for the part 3 test while earning money.
However, it’s important to make use of the trainee licence so that it genuinely progresses your teaching ability and to not use it as a means to simply earn a living. Think of the trainee licence as an extension of your training and not a job opportunity.
Part of the conditions of a trainee licence is that your trainer must provide an additional 20 hours of training, or that 20 percent of your lessons are supervised. Use this training or supervised feedback to ensure your instructional ability is progressing you towards passing your part 3.