What is the purpose of a control group in an experiment?
Suppose you have have 2000 people who are suffering from some disease.
You give a new drug to 1000 of these people and discover that after 5 years 65% of them are dead.
In order to suggest some possible significance to this result we need to know what percentage of people with this disease were dead after 5 years if they did not receive the new drug. These people who did not receive the new drug are your control group.
Obviously your satisfaction with the new drug will be considerably different if 98% of your control group has died in contrast to if only 15% of the control group had died.