Final answer:
The bold font used when Edith warns the patron with 'Hot plate, sir' serves to emphasize the importance of her warning to prevent the patron from burning themselves.
Step-by-step explanation:
The use of bold font in the text when Edith says, "Hot plate, sir," indicates emphasis. This stylistic choice is often used to show that the speaker is putting stress on certain words to convey an urgent or important message. In this case, Edith is emphasizing the temperature of the plate to warn the patron of the potential danger.