A CDT is a cohort of students that receive a year of cohort based training in the first year, before they start their DPhil, so the duration is four years instead of three years. If you start a DPhil straight away you don't have the year of expert training plus you won't necessarily have other DPhil students joining with you. Therefore, the benefits of a CDT are that you get to be with a group of like-minded students and you can have an additional year to bolster your skills while investigating what types of research areas are available, by reaching out to potential supervisors. But, for some people that's a disadvantage if they know exactly what they want to research and want to start straight away and complete their DPhil in 3 years.