The word "while" helps the reader determine that the character goes to the market at the same time the water is heating.
Explanation:
The word while in this excerpt gives the reader an idea of how long it actually took the water to boil. Since there was probably a good amount of water over a fire, the main character took advantage of the time to go to the market and purchase several things that she needed. The word while implies that the time the water took to boil was enough to let the main character go to the market, select their products and return to the house to get cleaned up.
