1) It's Cheap
Many people think that Film Photography is expensive. If you're going all out and developing your own photographs rather than taking the to a developer then it might well be expensive! However manual cameras are a dying breed and you can pick up most of the bits and pieces you need to get started, quite cheaply.
2) Manual Focus
It's the simplest reason, but it's so true. Manual Focus will save your life. Learning how to focus your camera the hard way will make you a much better Photographer in the long run.
3) Appreciate Modernisation
Spending hours in a darkroom can be fulfilling, I've done it, and I loved it. But it can also make you appreciate modern photography that much more. Yes it's great to use a manual camera here and there, but it's even better when you can just pick up, snap and post to instagram in a few short seconds. Yes, instagram! Manual Photography will make you appreciate what we have today, so much more.
Most of the pros of using a manual camera are that it's not all done for you. How easy is it to whip your camera into Auto and let it do all the work? Too easy. With a manual camera, you have the same ISO throughout a roll so you need to get it right the first time in order to get the best exposure.
5) Cost Effective
Not only is it cheap, as mentioned in point number 1, but having a manual camera will not tempt you with all the awesome and fantastic upgrades, add ons and other bits and pieces that modern photography now has. You'll save money, AND be an awesome Photographer, what more could you want?!
Manual Photography isn't for everybody. I will say that you'll need a little more patience than usual and an interest in getting results. You'll have to plan carefully to ensure that you don't make mistakes as manual is unforgiving. But it's worth it, and if you fancy learning the basics then why not give it a go? Check out This article for more detailed tutorials.