petri dishes are used to grow out spores from shrooms collected in the wild, it enables you to transfer non contaminated sections to another plate until you have a clean dish, with only mycelium.
It also makes it possible to identify more viable sections of mycelium. Also it can be used to clone mushroom tissue into a true mono strain, but I prefer a proper liquid cultre for this.
As for big flushes, you realy want to work from one of these clone isolates. Your first grow from multispore will fruit unevenly over a couple of days, with big variation in size and potency from shroom to shroom.
It is important to take some spore prints after fruiting for future use and storage in case of failures. Then to actualy make a liquid clone of the best fruit from the largest and fastest growing cluster, and do your future grows from that.
It is a simple procedure requiring only some spent coffee grounds, water and sugar. I'll talk you trough when we get there.