You could spend hours online reading about whether is was a good idea to write a treatment or outline before you begin to write a script. They are often good threads to read, but I get frustrated when you start seeing words like 'always' and 'never'; always write a treatment, always do this and so on. So here's what I think abut the topic...
It's over to you. That sounds trite but there's a deeper level to look at here; every process will tend to produce a similar product. By that I mean if you write a film straight after reading anything by a contemporary script guru it will tend to read like a version of their process. If you decide that you want to 'go auteur' and follow the method of Auteur X, then your next work is going to look like it.
So if you want to produce exceptional, non derivative, unique stories for the screen you had better have a non derivative and unique process that is entirely yours. And I don't mean whether you use a lined pad or a 3x5 card I mean the really hard stuff where you figure out what your movie is an why the hell you are going to spill blood over it.
'Writing a treatment' or not is a question to ask for each project as it comes up , why say that you always need to or not, this is just a straightjacket. We all probably need to do it (and pitches etc) because there are several ways that a story needs to be told for it to sell according to who the audience is at that moment (a producer, a pitch comp, a director, a signed actor).
But how you unearth, shape and mould your story... you need to completely claim that bit for yourself. This is part of the long (glorious) journey of being a writer, finding out whether you keep it all in your head, whether you are better with a strict schedule, at night, is your writing better when you are healthy, sad, broke, desperate and so on.
My experience is that the process is very different depending on movie type; I have written treatments longer than the script and sometimes I just have very large pieces of paper with dense writing over them which I guess is an outline. For plays I might have a one or two page step outline and deep character notes and pictures, but then again sometimes I just go back through my journals, pick out scenes and notes and jump in to something halfway through and trust the rest will follow. Sometimes writing the f**king thing is the only way to find out what the problems are, sometimes going back to cards is what you need. The only thing that I have found is consistent is putting together a playlist for the piece to listen to as I write and a couple of key images poised over the monitor.
This is the blood and guts of it, the rest is just a bunch of techniques that you can learn and apply when you feel that they are necessary to the moment.
What a great activity writing is :-)