Endorphins. Massage feels good and human touch has been shown to be beneficial in recovery from trauma. The massage itself may release endorphins which decrease pain and give you a feeling of well-being.
It helps. Depending on the type of injury, massage therapy may or may not be indicated. If a strain/sprain occurred, it may be helpful in that it might reduce pain and speed up recovery. There are different types of massage - light/relaxing, which may be especially helpful in acute/subacute recovery phase and deep massage that may be helpful in remote recovery period. That usually stimulates muscles better.