I can't answer this for yeast, but in the case of bacteria, moisture from the top of the plate would drop onto the plate and interfere with growth. I have some examples: 1. Assessing growth through plate counts -- dripping moisture might spread individuals and colonies making them impossible to count or changing the counts. 2. Streaking from mixed cultures -- moisture dripping onto the plate might lead to mixed colonies, thus interfering with being able to isolate pure cultures. 3. Preparing a disk plate for antibiotic sensitivity -- moisture dripping onto the plate may lead to movement of the disks or uneven diffusion of the antibiotic from the disks. Hopefully that helps.

Thank you Ellen Lark. Your deep insight about the topic is much appreciated

Yes,Ellen