Restaurants serve water with lemon for two main reasons; as a garnish and as a way to disguise the taste of poor quality tap water. A slice or wedge of lemon added to a glass of water is believed to be an embellishment.
As most cities have poor-tasting water, restaurants that serve tap water to their customers use lemon to mask this bad taste. In some cases, the addition of lemon slices to drinking water in restaurants could be done simply as a matter of habit. Servers are often trained to add lemon slices to teas and water to make it look more appealing.