A number of reasons OP. On my course there are mainly international students, some who use others':
-Some are simply super rich and have no motive to do their own work and can easily afford to pay others to do it
-Others are not so rich and are working crazy hours beyond what they are meant to be doing. They rather trade off working in a 'normal' job as opposed to doing their own assignments
-Main reason is many simply shouldn't have been allowed on to the course because of their grasp of english- but universities simply can't let those large tuition fees go.
Im not an international student but I have genuine reason to be using such services because of personal problems interfering with my studies.