They have to do it to gain more leverage over the display supply chain , to allow Apple to deal with price pressures in the smartphone and tablet markets without sacrificing profit margins.
The point I am trying to make is that Apple has to sell a TV, obviously to grow the top and bottom line and increase the footprint of iOS devices, but also (and probably more importantly) to gain more leverage over the display supply chain to protect the competiveness of the iPhone and iPad franchise.