clone only generally relates to strains that have somehow died out in seed form or never existed ie: original cheese, there are only mother plants that exist ( ithink it was a one off that was cloned) and no male plants to pollinate with so it would have to be crossed to get seeds so would not be the same strain so it becomes a clone only strain,
or you get strains that are crossed or are jus phenos usually purple strains like purple urkle where seeds were never made as it isnt possible as only a female purple urkle exists so it again has to be crossed with another strain to produce seed this strain was called querkle
there are loadsa reasons why a plant could be clone only and it generally depends upon how the strain came to exist in the first place