Although, here I am, starting with the short version: it will force you to take your writing seriously.
Attending takes a serious commitment of time and money. (Although I will say right now, and I mean this, if the money is the only thing holding you back from applying, apply. Worry about the money when you have to - there are grants and scholarships. Get in first, and then figure out how you will pay for it.) So by applying, and deciding that you would go, if you got in - that you would put all the rest of the parts of your life on hold for six weeks - you have made a decision about how important writing is to you.
Being there is hard work. Ideally, you workshop a story a week, so that you can take advantage of having a critique from each instructor. Writing a new story a week is difficult on its own (if you don't believe me, try it sometime. Take six weeks, and write six short stories.) but you are also critiquing an average of 3-4 stories a day. This is also something that takes a good deal of time and effort, because you try to do a good job on these, to really think about how the pieces of each story do or don't fit together, and to give your classmates useful notes. This may be the first time you've been in a critique group, or the first time you haven't been the best writer in the room. Those things are an adjustment.
Critique sessions ran 3-4 hours in the mornings, and there would sometimes be addition presentations by the instructors in the afternoon. You meet with each instructor for a private conference after your story has been workshopped. Some days there are guest speakers.
Sometimes you eat. Sometimes you sleep. But really, what you do is think about writing.
Not just about how to write, or how to write better, though there is a great deal of that, but about why you, you personally, write. How do you see writing fitting into your life? Do you want to pursue it as a career? What kind of shape do you hope your career has? What kind of thing do you want to write, and why do you want to write it?
And you get to think about these things with a group of other people who care, and passionately, about the same thing you care about. Who are working just as hard as you are, and struggling with the same things. So you take the idea of writing seriously, and you learn the skills and gain the support system to take writing seriously when you reenter the real world.