Dietary fats make food tasty they often improve the texture of food as well as flavour and smell - they make food more appealing. In the UK, the Department of Health suggests that no more than 35% of total calories should come from fat. In the US, recommended fat intake is 30% of total calorie intake. In reality most Western diets derive at least 40% (and sometimes a lot more) of their energy from fats.
Due to its high calorific value (1 gram of fat = 9 calories) it is easy to consume too many calories when eating fatty foods. Unused calories can be stored by the body as fat and will cause weight gain.