Different phenotype. Different phenotypes have different characteristics. Small to tall, bushy to thin etc etc... Google it, it's fairly easy to understand.
Unless you want to try a variety of strains you'd normally grow 10-15 of one strain (or even more) from seed and slowly single out the best plant then clone from that.
Its still easy to grow different strains or even the same strain from seed unless you're growing on a large scale, you can always raise pots if you are restricted for space to LST. Or if you have the space LST and super cropping and supplementry lights are always options.