Placing ginger slices on top of steamed crabs has the following benefits. Firstly, ginger has deodorizing effect, which can reduce the fishy smell of the crab. Secondly, ginger also has a flavor-enhancing effect, which can increase the aroma of the crab. Moreover, ginger helps to promote digestion and increase appetite. Therefore, placing ginger slices on top of steamed crabs can enhance the taste and flavor of the crabs.