A class that implements Comparable is sortable. It can be sorted according to the implemented compareTo method using the Comparable mechanism. This is called the Class's natural ordering.
Suppose you want to sort objects of this class based on some other criteria. Then you can create an implementation of a Comparator and use that. You can create yet another Comparator to sort it based on some other criteria.
Also when you want to define a sort order for a non Comparable object (i.e has not implemented Comparable) you can use a Comparator or many.
For example for a Job object having members jobID and priority, two Comparators can be implemented, one sorting on jobID and the other on priority.