kind of a strange question you are asking here......
Do you mean why do you need fat in your diet? If that is the case, becasue fat serves many functions to the cells in your body, such as insulation, protection etc. Also, fat is the building block of hormones.
Or, are you asking why you need to get fat in order to add muscle? Well, the above answers still stand. As well, it is difficult (pretty much impossible unless you are using steroids) to gain any sort of muscle mass without gaining weight. Unfortunatly, with that weight gain will come some muscle gain and some fat gain. You need to try and limit the latter and improve on the former as much as possible.