If you're trying to create a work of art by throwing wads of mud at a wall (and let's not even get started on why you might want to do such a thing), you could draw the design first and toss the mud inside the lines or you could toss the mud first and draw the design around them afterward--but either way, the lines give shape and purpose to the random splats.
I've seen students draft papers using either method: create a clear, compelling, specific thesis statement and then arrange the points in the argument to fit within the lines, or toss a bunch of semi-related ideas on the page and then go back and write a thesis that specifies the connection. A diligent writer committed to revision can make either method work, so part of my task as a writing teacher is to figure out what kinds of writers I have in the room and guide them toward success.
Me? I'm a little of both: in the early stages of a project, I'll toss a bunch of ideas against the wall and see what sticks, but as soon as the ultimate shape of the essay starts revealing itself, I draw the lines and write the thesis. It will surely get revised along the way, but I need structure to guide my writing.
But what can I do when students want to toss a bunch of mud clods against the wall, draw a vague, squiggly, illegible line around them, and then declare the work a masterpiece? I offer suggestions on drafts and require revision and I frequently offer sample thesis statements to serve as models, but I can't do the thinking for them.
Which is why it's so hard to answer the question "What should my thesis say?" If a student has put in the thinking required to determine how various ideas are related and has a sense of purpose, the question is simply asking for help putting that purpose into words. I worry, though, about the other kind of student, who hasn't done the thinking and doesn't have a clue about connections, and if I ask what purpose the essay is pursuing, the response is something like "I want to get a good grade." (Don't we all! But what does your paper want to accomplish?)
Some days I wear out my eyeballs staring at clods of mud thrown randomly at a wall--but when someone finally draws a line around it and reveals the design, all I can do is applaud.