I could be wrong but I think clone only strains are usually strains that are very unstable/unreliable when bred from seeds. And by unstable I mean they have a plethora of phenotypes that are produced when grown from seed. Stabilizing a strain can take many many generations of breeding, selecting desired phenotypes, re-crossing them over and over until there is a good chance that the seeds will all produce the desired phenotype. Now imagine two strains are crossed for the first time, 100 plants are grown from the seeds, and ONE of those plants is an amazing phenotype that is stronger, bigger, and more potent than the other 99. Now, until someone spends the time to refine the strain, do all the cross breeding, selecting, etc. (A process that can take years, Snerval is a good example of this, a f7 that was crossed and recrossed 7 times, each time selecting only the desired pheno, until stable genetics were possible), until all that is done if you want to grow that specific phenotype of that specific strain, you either need to take a cutting from the plant, or cross the parents again, grow out several dozen plants, and hope you get lucky with the phenotype. Until stable genetics are available, the strain is for all intents and purposes, "Clone Only". By cloning, and then cloning the clones, a single standout phenotype can be spread around without the need for the long process of stabilizing the strain.