The StateVector and DensityOperator classes (and their non-orthogonal counterparts) exist for two main reasons:

As interfaces to Types::StateVectorLow and Types::DensityOperatorLow, respectively, which are more convenient to use on higher levels of the framework, e.g. in scripts and in quantum trajectories. This is especially because while blitz::Array uses by-reference copy semantics and expression templates, these classes use the more usual by-value copy semantics and normal semantics for arithmetic operations. This means, however, that copying and arithmetics should be used judiciously, if possible only in the startup phase of simulations.
As implementations of the LazyDensityOperator interface.