For the same reason your GP doesn't do brain surgery but. if you needed such an operation , it would be useful to know when the brain surgeon was available to perform it! . A solicitor doesn't normally conduct advocacy in the Crown Court (there are rules about that). So a barrister is employed in your case. He or she will already have other cases on various dates. Therefore, though their attendance may not be necessary at a preliminary stage, it's important to know on what dates in their diary they will not be free to act in this trial