I wrote my first screenplay on an IBM Selectric--I suppose that's a bit of a transition between paper and computer. Both the typewriter and the computer fix the writer on the notion of the page; unless you write a lot with a pen, you're not seeing a page of handwritten text as a page of a printed script--you don't know exactly where you are in the 100-120 page finished product. This can be limiting or liberating (or both). In theory, in this draft you're focusing on plot / story rather than structure (though most of us have internalized a sense of structure anyway).

The computer (as opposed to paper or typewriter) forces a number of choices on the writer (even if the writer is using the standard defaults of whatever screenwriting software). In the case of a first draft in particular (if the writer hasn't already created a detailed outline or synopsis, i.e. is starting with an exploratory draft), then not having to worry about margins, dialogue length, and other formatting decisions until later in the drafting process may be liberating as well.