Why will massage therapy help accident injuries?

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.