Weber doesn't say what to use it on, but I am sure we can think of something. 
Creamy Horseradish Sauce
3/4 cup sour cream
2 TB. prepared horseradish
2 TB. finely chopped fresh Italian parsley
2 tsp. Dijon mustard
2 tsp. Worcestershire sauce
1/2 tsp. kosher salt
1/4 tsp. freshly ground black pepper
In a medium bowl thoroughly mix the ingredients. Cover and refrigerate until 30 minutes before serving. Makes about 1 cup
Source: Weber's Charcoal Grilling cookbook 2007

Creamy Horseradish Sauce
3/4 cup sour cream
2 TB. prepared horseradish
2 TB. finely chopped fresh Italian parsley
2 tsp. Dijon mustard
2 tsp. Worcestershire sauce
1/2 tsp. kosher salt
1/4 tsp. freshly ground black pepper
In a medium bowl thoroughly mix the ingredients. Cover and refrigerate until 30 minutes before serving. Makes about 1 cup
Source: Weber's Charcoal Grilling cookbook 2007