I took a look at that list on the wiki. Just curious, but why would we need petri dishes as home brewers?
To build on Fly's answer: the plate is often used as a way to propagate new-to-you yeasts or to reculture existing ones.
For example, you do a yeast trade with someone on HBT. You get the vial or plate in the mail and use it to streak your own plate. It verifies the culture made the trip through USPS and, as Fly noted, will allow you to pick up a single culture off the plate. If you're interested in teh details
google "quadrant streak" . It's actually a neat idea and is, as a programmer might say, an elegant solution.
Many folks store their cultures on plates. I find it easier to use plates as an intermediate step on the way to slants or water suspensions. I run into problems with managing the humidity on plates, and the surface area is so large (and my house is so dusty) that I find plates pick up contaminants easier. I work around this by making and inoculating multiple plates any time I am working with them. In theory one should be able to harvest a culture from a plate with a contaminant
way over there but I just use a different uncontaminated plate.
