Jail is not free. Lawyer fees, court fees, if there's a victim other than the state you'll be ordered to compensate them, when in jail you rely on outside money for all your little extras, a job is more likely to take you back if you go to rehab, no police records etc etc etc. Also, jail is full of drugs so if you plan on getting sober jail is really not the best place to do that.
To answer your first question: no, you don't need rehab if you already know all the keys to get sober on your own. Get as far as possible from civilization for a three weeks. Somewhere very desolate. Cut ties for good with everyone that is related to the substance you want to quit. Get rid of everything that reminds you of your habits. The first 3 weeks are the worst'. After you come back it's crucial you have a plan to avoid falling back. Get outside as much as possible. Long long walks. Getting involved in something that will eat up your time and fill the void.
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