TCP Connection Management

In TCP, each SYN and FIN segment consumes one unit in sequence number space, even though no actual user content or data is included in that segment. For example, if first sequence number is initialized to x. SYN packet will have sequence number x, and the actual data will start with sequence number x+1. Similarly, if the last data transmitted has sequence number z. FIN segment will have sequence number z+1.

Is this necessary? If so, give an example of an ambiguity that would arise otherwise; if not, explain why.