Leigh Whannell and James Wan needed a hook that extended beyond a simple haunted house in the first Insidious, and they found it by fleshing out the universe with an entirely different alternate dimension.
We have seen ghost stories told many different ways over the years, but astral projection involve demons and spirits invading a familiar environment like a house or an apartment, Insidious adds to that mixture by also featuring a realm that our heroes can journey into in their quest to beat the demons.