I wrote a long post about this but computer went pffft.
Moving for the sake of moving isn't necessarily useful, but moving teachers to different classes is seen as best practice because it adds to the overall stability of the school. This normally leads to secure learning for the children, and to career advancement or teaching enhancement for teaching staff. Remember we're speaking of Primary School level here. (Well, I am.)
Changing year groups helps teachers better understand what they need to do to prepare a group for the next year, or to better understand the difficulties in preparing a group for their own year.
Changing also allows teachers to prepare professionally (such as those who wish to move toward Deputy or Headteacher positions--it's incredibly useful to understand each year group's needs).
Changing year groups also allows a school to have staff with some experience in place, if staff turnovers get out of control for whatever reasons, rather than having to narrow the search for new staff by asking for/hoping for specialised year group teachers.
For the children, it can be a breath of fresh air. My children are close in age and tend to tell each other what to expect from the next year's teacher. If that teacher is different, it gives the younger ones an opportunity to experience the year on their own, without the older sibling filter.