As a teacher, you do not complete all your learning once you have finished your university degree. A lot of learning is actually done whilst you are teaching on the job and continued throughout your career. As teaching methods and the curriculum are adapted over the years, it is vital that you also adapt your teaching style. It may be that new ideas and research comes out which is going to be useful in teaching a certain topic or changes that are going on in the world become important and need to be included in what is being taught. It may be that you need to attend another course to learn about these new methods or topics. Often teachers are sent on several different types of courses throughout the year to help them continue to grow and be the best that they can be.
It is important that you attend as many of these courses as you possibly can as you will often find them really useful and a great space to speak to other teachers about difficulties they are having and how to over come them.
If there are several members of staff that need to attend a course, then the school may arrange for someone to come in rather than them all going out.