After two years of treatment, it sounds like you are almost done with your orthodontic treatment! Elastics/rubber bands are used throughout treatment at various stages for differing purposes. Towards the end of treatment elastics help to finish the bite so that the treatment result is stable. For example, if the top and bottom front teeth are touching a little too hard when you bite down compared with the back teeth, after treatment the upper front teeth may start to space out and move forward from the excess pressure. Wearing elastics from the lower front teeth to upper back teeth applies pressure to slightly move the lower teeth back. This fine tunes the bite so that the front teeth do not have an interference when you chew that may cause unusual wear or spacing. The way that you have been wearing elastics throughout treatment commonly changes in the final few finishing appointments.