In a country where weed is grown out doors you never know which male has pollinated your females.
Cannabis is pollinated by the wind and can travel for miles before it hits your female plant.
This causes a lot of of diversity in the seeds that you get, when the seeds are grown, some may be tall and lanky , some short and stout.
The multiple planting allows the grower to choose the type of plant they want to grow and ditch the rest.
Selective indoor breeding of plants produce plants that are a lot more homogenous and display almost no variation o have just a few dominant pheno types. These are the strains that you would find in a seed bank/ seed selling site.
We have the luxury of not having to grow multiple plants to find the best plant as we know the genetic make up of the strains we buy.
For example we know that Lowryder is Williams Wonder x Northern Lights #2 x Mexican Ruderalis
Lowryder has the following know phenotypes
1.: short, compact, fastly blossoming, indica-dominant Phenotype #2
2.: long, stretched, slowly blossoming, sativa-dominant Phenotype #1
We also know approximately how tall it grows, that it autoflowers and how long it's flowering period is.
Most of the guess work has been taken out by the breeder, so there is no need to grow multiple plants to selct which ones survive and which ones get chucked.
On occassion a special pheno may be found which may be of interest to a breeder who is looking for certain traits to use in a cross.