Why would a test on rats include a group treated by a placebo?
Because the mere act of telling a rat that you are giving it medicine for diabetes can cause the rat to have a psychological response that improves their condition.
Therefore you need a control group. You must have a second set of rats that have been lied to and told they are receiving diabetes medication but in fact are receiving a placebo.
And don't ever write your password down in a rat lab, because the rats let themselves out at night and read the lab reports. That uncontrolled feedback can really skew the results.