When my son was 18 he enrolled in a 28 day rehab facility. The director met with us and told us that it was a very good sign that our son made this choice. He also gave us the facts on recovery. He said that some users do find recovery but many will modify their use at best, and take more time and experience to get to a place of finally quitting.
It's still a good place to be and like BigSis said they learn while they are there. The tools are made available and the more exposure to recovery a person has- and the more someone realizes the truth- the sooner they may decide to change.
My son stayed out of trouble for about a year afterwards and has been in and out the same halfway house several times in addition to successfully completing their 6 month program...twice. He stayed on for 3 months as staff the last time and when he moved out began his last 'run' from which he ended up in jail. This time he chose to do a hard program for a longer period of time- in a plea exchange. He could have easily been back out on the streets using- but chose a better path.
He has been clean 14 months and will be moving out of the court ordered halfway facility in a couple weeks.
I agree with what Rose said- that rehabs and halfway houses are not always necessary. What is necessary is willingness and whatever it takes to get to that place.