It is not, but you may reach a point when reducing calories is no longer practical or may be extreme, in which case adding in cardio and maintaining or adding back a bit of calories will be better.
Moreover, there are much more benefits of cardio than just caloric expenditure, such as increased metabolic rate, improved nutrient partitioning, and increased work capacity.
It is smart to start off conservatively, with minimal amounts weekly. As fat loss begins to plateau, additional sessions should be added and so forth. Variety is key as well, not allowing the body to become adapter to any one protocol.
I know from personal experience, I have much better results during a cut when I do have some form cardio in my routine.