I went to a 6th form college and my daughter has too. Often choice is the major reason, but my daughter had no choice.
Visiting lots of sixth forms and colleges, it was obvious that they differ very much in pastoral care and parental feedback. One seemed very geared to the students being independant (this is the one she has gone too - no parents evenings, all student one to ones for feedback) but others handheld a bit more, which would suit some students better.
By moving they offered change and they offered a huge choice of courses, which is obviously good for those who need it. Being able to reinvent oneself is no bad thing after five secondary school years.
Travel and adjustment are usually the disadvantages.
In my daughters case, another disadvantage is the lack of extra curricular, but some sixth firm colleges excel at this. To make a firm decision, you really need to visit them all, read their prospectuses so your teenager can make an informed, tailored decision.