I really can't get my head around why they are starting in the middle.
This explains it quite well: Another Character From Later Books Is Cast In DARK TOWER!
The first film will tell Jake's story from the first three books. That's very much how the trailer plays out. The trailer starts out from the perspective of our world, rather than Roland's. It's probably both an artistic choice and a way to condense the books as a whole. The first book leaves you with a lot of questions, the last chapter is quite puzzling - maybe not such a good idea if you want to build a franchise and want people to come back (people want to have some idea of where something is going) - plus eight books, comics, short stories, is a lot to film literally.