Rubbing cucumbers before eating is a common practice in some cultures, and there are a few reasons why people do it:
To remove wax or pesticides: Cucumbers are often coated with wax to help preserve their freshness and appearance. Additionally, conventionally grown cucumbers may have been treated with pesticides. Rubbing the cucumber under running water or with a cloth can help remove any wax or residue from the surface of the cucumber.
To improve texture: Some people believe that rubbing cucumbers can help to improve their texture and make them more crisp and crunchy. Rubbing the cucumber with a little salt can also draw out excess moisture and make them less watery.
To release flavor: Rubbing cucumbers can also help to release some of their natural juices and enhance their flavor. This is particularly true for varieties with thicker skins, which can be bitter or tough.