What is the difference between $CMD and eval $CMD?

Question

We had code like this in a shell script:

CMD="docker container create \
    --name $CONTAINER_NAME \
    --network $NETWORK \
    nginx:1.17 \
    nginx-debug -g 'daemon off;'"
$CMD

when we executed the script, it did not work as expected. The container gives following error:

nginx: invalid option: "off;'"

Then we changed to:

eval $CMD

in the bash script and then it started working! I'd like to understand what is the difference between the two.