The boys becoming corporeal also means that they will be able to be seen by the living all the time, not just when performing.
There's clearly already something "special" about them, as Caleb notes, and they can be heard by the people on the beach despite "This Band Is Back" not being a real performance. Them taking on corporeal form will allow them to communicate directly with the living, having powers similar to Caleb's.
Alex will break whatever hold Caleb has on Willie's soul
If Julie could break the band's curse with The Power of Love, Alex will be able to break Willie's the same way.
