Generally people write a letter of intent after they interview at a school, because they have heard about the curriculum and met the students and can more accurately describe how they feel they would fit in to, contribute to, and benefit from that school. A "good reason" is any reason that you want to go to the school, although generally people focus more on why the school (curriculum, student body, opportunities) than the location, although location can be included if the letter includes other reasons why you'd want to go to that school and you have a good reason to prefer that location (family nearby, offering a strong social support system, or opportunities available in that location) rather than just saying "I really love Boston b/c I'm a Red Sox fan".
Keep in mind that a letter of INTENT includes a statement saying "I will definitely attend your school if you accept me". A letter of interest just explains why you're interested in the school and how you'd fit in there. People on SDN will argue until the cows come home about whether or not a letter of intent will help your application, or whether or not it's "binding". I think the general opinion on these two matters is #1 - the weight a LOI will hold depends upon the school, but it can't hurt unless your letter is poorly written or includes ridiculous reasons to want to attend the school, and #2 - the LOI may not be officially binding, but if you're going to write one, you should be prepared to attend that school, regardless of wherever else you get accepted or what your financial aid package looks like. Doing anything else is unprofessional, and you never know what the consequences could be.
To the OP: you can write a letter of intent pre-interview, but it will probably not get you very far. Adcoms are not going to take your letter seriously if you haven't even been on the school's campus, unless you have a very compelling reason (ie, your spouse goes to school there or works there and can't transfer). You'd probably be better off writing a very enthusiastic letter of interest, including any updates you might have, and hoping for an interview. Post-interview, go ahead and write the letter of intent. However, before you put the effort in, I'd find out what the school's policy is on letters like that. For example, I know that Stanford won't accept any supplemental materials once your secondary has been submitted and before you're offered an interview. Perhaps you should call the school and say something like "Hi, I'm really interested in interviewing at school XYZ and I'd like to write a letter discussing my interest. Could you please tell me who I should address the letter to?". Good luck.
