In an experiment on petri dishes, what purpose does plating the yeast serve?

Yeast can be easily plated or grown in petri dishes. Plating of yeasts can serve a number of purposes:
1) It allows one to demonstrate that yeast can grow in petri dishes. This would be especially relevant to students.
2) It allows one to demonstrate the proper protocol for yeast plating (or streaking).
3) It allows one to isolate a particular strain of yeast (through successive plating) from a mixture of microbes.
4) It allows one to create a yeast bank (growing a large amount of yeast and storing it for future use).
5) It allows one to calculate the amount of yeast present in a sample. This is done by using a definite volume of sample for plating and counting the resulting yeast colonies.
6) It allows one to isolate single colonies for further plating.
7) It allows one to calculate the doubling time (time required to double the number of cells).
8) It allows one to draw the growth curve, including the lag phase, exponential growth phase, stationary phase, and death phase.
Thus, plating of yeast can be done for a number of reasons. Students, researchers, and brewers also routinely plate yeasts for their specific purposes. Interested people can also grow yeast at home.
Hope this helps.