IAAL and I did a lot of patent prosecution for one phase of my career, about 12 years ago. The top three reasons there make sense to me. The justification I remember as being the main one is the doctrine of claim differentiation: the existence of one or more dependent claims adds support for a relatively broad interpretation of the independent claim, i.e. the independent claim is presumed not to require the additional limitation(s) recited in a dependent claim.