I assume you have various individuals/groups who have an interest in the systems you administrate. Users, developers, etc. Also regulators. Don't forget the utility of a good documentation system when the auditors come around*. So you need a process to keep them informed of the upcomming system changes. So they can ensure that their product or process isn't going to be broken by a change.
If you have relatively few of thes interested parties, the communications could be mandles manually and by you. If that community is large, the procedures need to be formalized and possibly automated. Having a CAB to represent your user community can offload the communications task from you. At the expense of some paperwork.
On the other hand, I've worked in organizations where the CAB was a make-work task for a few layers of management. People whos only other job prospects are standing by an off-ramp with a cardboard sign*.
*At one of my previous jobs, this was the acid test of the utility of our CAB. I had to fill out stacks of paperwork and await their blessing to make a change. But strangely enough, whenever the FAA came around, they were nowhere to be found. I had to walk the auditors through our systems myself.