well john i am very happy for you. you've made a good decision. you are giving yourself the gift of life. here's what rehab did for me:
educated me about the disease of addiction and alcoholism.
kept me sober long enough for me to remember how good life can be clearheaded.
introduced me to the literature of AA.
gave me hope i could get sober.
fed me a balanced diet. when i drank i didn't eat or had any liquid other than alcohol. this could be days.
taught me the importance of staying away from ppl, places and things pertaining to my drinking career. it is/was too much of a temptation.

these are only a few benefits i had. there are so many.
i wish you well on this journey and please let us know how you're doing.
between my experience at rehab and attending AA thereafter i have 23 yrs. sober. there's hope for you too.