a silly question: to use Matlab on GPUs, we need write .cu files and compile it using nvmex. Then why wold one use Matlab, if he already knows how to write .cu in C language? Given a task, he may write a cuda program rather than matlab program to achieve it.
Actually you use the GPU from matlab. Reasons:
legacy code in matlab that you want to speed up (often only small parts need to be converted)
matlab is great for prototyping.
visualisation of results is easy in matlab