please use the search option on the shroomery first.
Rye to rye is usually done for making more spawn. Spawn needs to be sterile to prevent contams later on.
Rye to straw is done only to colonize the straw for producing fruits. Usually the straw is pasteurized, so it's not sterile at all, it just gives your spawn an advantage.
The reason to use spawn is to colonize the medium extra fast by providing allot of inoculation points(read grain kernels)in order to make sure that only the wanted fungus will win the race of colonization.
hope this helps
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