Aside from making conceptual sense, using the original value has the nice property that there's no dependence between the calculation of separate pixels, so the filtered image can be computed fully in parallel. GPUs are very good at this, which obviously makes them useful for graphics rendering, but also at other tasks that involve the same mathematical operation, such as deep learning (the first few stages of a convolution deep neural net are just the application of specially tuned convolution kernels).
