In 99 % of the cases, a yoga class is finished with some relaxing time in savasana. Savasana I the time when the practice really settles in the body, and you are given some time to come back to yourself. Savasana is the reward for some great work.
Yoga is about balance. In savasana, you are given time to balance the body, after work (ha) with relaxation (that). Otherwise, the work would just be some ha-ha work, and the practice will lose one of its greatest benefits.
Even though it looks simple, savasana may be one of the hardest poses to actually do. Too often I see people leave just before this pose. If you do have to leave the class early, make sure to quit your practice some minutes earlier and always make time for savasana.