Answer:
In order to get more young people for the war.
Step-by-step explanation:
The congress created the law of lowering the age of voting in order to get more young people for the war. The person who can vote can also participate in the war. The congress needed men for the Vietnam war so for that purpose it lower the age of voting. The immediate effects of this lowering the age for voting is that the United States have more young people for the Vietnam war and more soldiers provides their services in the war.