I'll go for - immediacy. It's happening right now, in front of you, and you the writer have get it all down on paper swiftly and succinctly if you're going to keep up...

You can only show what's going on in the moment.

It's a great reminder of how the audience will experience the film. The movie happens for them in the present. So we write in the present tense.

All three of you got the main point: There is an immediacy to the present tense where things are unfolding NOW. But the implications go beyond that. There is also writing in the NOW where we experience the action as it plays out. Then the characters making choices and saying things in the moment, which gives them a vitality and spontaneity not present in the past tense. If we can embrace the power of the Present, we can imbue our stories with a special kind of energy.