Knitting fabric that is waterproof for fitted sheets offers several advantages. Here are a few of them:

1. Protection from spills and stains: A waterproof fitted sheet can protect your mattress from spills, stains, and other forms of moisture. This is especially useful if you have children or pets in your household, or if you enjoy eating or drinking in bed.

2. Hygienic: A waterproof fitted sheet can help prevent the growth of bacteria, mold, and mildew in your mattress by preventing moisture from seeping into it. This can help to maintain a clean and hygienic sleeping environment.
   

3. Breathable: Some waterproof fabrics can be designed to be breathable, allowing for air circulation to prevent heat buildup and discomfort during sleep. This can help you to stay comfortable and cool throughout the night.

4. Easy to clean: A waterproof fitted sheet can be easily cleaned with a simple wipe-down or by throwing it into the washing machine. This can save time and hassle compared to cleaning a mattress, which can be difficult and time-consuming.

5. Durable: Waterproof fitted sheets made from high-quality materials can be durable and long-lasting. This can save you money in the long run, as you won't have to replace your fitted sheet as frequently.

Overall, a knitting fabric that is waterproof for fitted sheets offers several practical benefits that can help to keep your sleeping environment clean, comfortable, and hygienic.