Why do universities offer full fund positions to international student? Many top universities in north America accept international ph.d. students and pay their tuitions, insurnace and other funds. To get visa, international students should convince the officer that they will go back to their home countries. I am wondering why universities should accept international students and what are the benefits for universities and the destionation universities?
Universities use Ph.D. students as teaching or research assistants. Essentially they are hiring you, with education being a part of payment. 
Moreover, if a Ph.D. student becomes a famous or at least succefull scientist, it is very good for image of a university.