Because the OP specifically mentioned submarines, and I was a U.S. submarine officer once upon a time, I’ll chime in.
Others have already mentioned the fact that ballistic missile submarines are the sole platforms most likely to survive a first-strike nuclear attack. The whole rationale for MAD and a credible nuclear deterrent is to ensure that an enemy is confident that he cannot take out a significant fraction of our nuclear forces. Air Force missiles sit in their silos in known locations, and bombers sit on runways and air bases. Submarines at sea cannot easily be located, much less destroyed by any potential adversaries.
As far as attack submarines go, realize how devastating the relatively crude WWI and WWII era submarines and U-Boats were in past conflicts. The U.S. is a maritime nation. Much of our trade is shipped to and from overseas ports. Without a strong Navy, hostile nations could blockade or disrupt this shipping, and the easiest way to disrupt shipping is with submarines. Finally, the best way to defend against a hostile submarine is with another submarine.
In a real no-holds barred shooting war, surface warships would last about 15 minutes. Between supersonic anti-ship missiles, maritime nukes, and enemy submarines, any surface ship floating is a sitting duck.
Based on our location in the world and our neighbors, the U.S. has little to fear with respect to a land invasion. It is far more important to protect our maritime trade, and defend against an (unlikely) invasion by sea. People forget that the U.S. actually used to build coastal defense gun batteries to defend our coasts. With our Navy and our submarine force, any potential belligerents would never make it to our shores in the first place.
With all that being said, I think that the size of our military could be reduced. I also think that money could be saved by taking a hard look at military salaries. While I think that anybody getting shot at should be well compensated with increased combat pay, there are a lot of people in our military serving their time and getting paid far more than is warranted. Between bonuses, untaxed allowances, and superb benefits, military personnel are very well-compensated–too well compensated, IMHO. I left the service eight years ago, and took a drastic pay cut, even taking a job as an engineer. Correcting for inflation, I didn’t actually catch up to my military salary until this year. Looking at current military pay rates, I see that they have increased at a faster rate than that of inflation since I got out.
Finally, I think that it is far too easy to go to war today. Congress needs to take back its power to decide whether or not our country goes to war. We have wasted an ungodly amount of money in the useless war in Iraq, IMHO.