Fighting a guerrilla style war benefits the North Vietnamese government because they knew their environment well (while their opponents did not), they were not as well-equipped or experienced in fighting as their opponents, and they were in a defensive war.
The motive for the North Vietnamese government to order conventional offensive attacks in South Vietnam is because they now had the advantage and could fight in a conventional manner, they had the supplies and military experience to fight in a conventional way, and it will help them look better and more legitimate in the eyes of the rest of the world as well as the people of South Vietnam.
(This answer is assuming that the second part of the question is referring to the assaults on South Vietnam towards the end of the war. If not, please tell me in the comments, and I can give an answer for earlier in the war)
