Purpose of using Nextval while creating the sequence - oracle

Purpose of using Nextval while creating the sequence.

NEXTVAL returns the next value of a SEQUENCE number. A reference to NEXTVAL must be preceded by the SEQUENCE object name

NEXTVAL Example

If DEPTNO is a sequence number, we can create a new department name with:
INSERT INTO Student
(rollno, name, age)
VALUES
(rollno.NEXTVAL, 'Prashant', 15);