The design of having a layer of clay on the sponge or within a clay block serves several practical purposes in car detailing:

1. Ease of Application: The layer of clay embedded in the sponge or block makes it easier to handle during the detailing process. Unlike traditional clay bars that require constant kneading and folding to expose clean clay, clay blocks with integrated clay allow for continuous and efficient use.

2. Uniform Application of Pressure: The sponge or block provides a more uniform application of pressure across the surface of the vehicle compared to traditional clay bars. This helps in evenly distributing the friction needed to remove contaminants without causing marring or scratching the paint.

3. Durability and Longevity: Having a layer of clay integrated into the sponge or block enhances its durability and longevity. Traditional clay bars can sometimes break or become unusable if mishandled or dropped. In contrast, clay blocks with embedded clay are less prone to breaking and can withstand multiple uses.

4. User-Friendliness: For beginners or DIY enthusiasts, clay blocks with integrated clay are often more user-friendly. They require less technique and skill to use effectively compared to traditional clay bars, making them accessible to a wider range of car enthusiasts.

5. Cost-Effectiveness: While initial costs may vary, clay blocks with integrated clay can be more cost-effective in the long run. They typically last longer and require less frequent replacement compared to traditional clay bars, providing better value for money over time.

Overall, the design of having a layer of clay on the sponge or within a clay block enhances usability, durability, and efficiency in the process of removing contaminants from a car's paint surface during detailing.