Can I put a different slant on this. If the Monarch is the head of the armed forces, and they are her Majesty's forces, than means that the army aren't independent, as they are in some countries, or under the ultimate control of the government. As we are a"polite" non rioting, non coup masterminding and so on nation, this may not be particularly important now but it would have been in the past and, who knows, might be again in the future. While the Queen and her heir hold the power, other people can't.
I know I haven't explained that well...does it make any sense?