It helps the language feel more natural to have words that have multiple related meanings. In English, for example, "cell" can mean a prison cell, a biological cell, or a cell phone, all of which derive from the original notion of a small chamber. It does make things a little more complicated, but that's languages for you. (At least no word in Klingon has as many meanings as something like the German "Zug"...) It's also more of a challenge to the language designer to see what existing words could be used to express a concept, and provides an opportunity to think of clever ways to have related words.