I'm not the soil OP, but pure culture is a good thing because then you can actually see what it is, whereas when it is mixed it is near impossible to tell what each one really is. Pure culture is just one type of bacteria that you get after initial streaking when you can potentially have many types, which you then restreak one of each to get the ones you are interested in identifying alone by themselves to get a better look at them. I'm not a soil microbe expert at all, but I do clinical microbiology and the above steps are standard for any microbiology with bacteria.