Actually, they are used because news directors want to be able to break up the show with outside broadcasts to avoid the news simply being two people talking from a desk.
In my opinion, that's the type of thing that a news director would say to justify the expense of a new truck. I am not trying to be snarky, that's my take based on previous work experience for a morning newscast and a long-time interest in the business of news.
I would add that modern broadcast news has always relied on other types of imagery to break up the anchor shot, chiefly prerecorded reports from the field (which do not require a truck -- at my station the photographers and reporters used ordinary cars) and weather screens. The SNG trucks are nice to have when something big does break, but most of the time they are simply not needed ... hence the contrived uses and excuses.
I think the sharp decline in costs for competing technologies are forcing a rethink of newsgathering costs. In recent years, helicopters have been replaced by in-studio CGI based on traffic maps and (on occasion) drones. This trend will accelerate, although I think one of the main expenses -- "talent" -- won't go away. Viewers do like personalities and on-screen charisma.
