Read on if you are interested why it matters.
The US Army is a branch of the USA military. The US Army is not a costumer of the F-35. The F-35 is a joint project between the U.S. Air Force, Navy, and Marine Corps. That's 3 other branches.
Now one can say that this is a silly interjection and I should just see "US military" every time I see the word "US army" in the GP comment.
But, in the case when we are trying to understand the F-35 project understanding the crazy force structure of the US military is a must. Why? Because that structure left an indelible mark on the airplane!
Very roughly: The Air Force is the folks with the airplanes, the Navy is the one with the ships and the Marine Corps are the troops on the ship. But turns out in modern warfare you can't do naval warfare without having proper air cover, so the US Navy also got airplanes. And historically the marines were left in the lurch one too many times so they insist on having their own transportation, so they have ships, and also airplanes. (Very rough explanation, bear with me.)
You can see that the branches are somewhat converging, will we end up with 3 carbon copies of the same thing? The legislators asked the same question and put in the law limits on each branch. There are experts on what the particular limits are and etc etc, what matters to us in terms of F-35 that the Marine Corps can't have ships bigger than X. So they can't just buy a conventional aircraft carrier. Therefore the marine's airplanes have to take off vertically.
This means that when this 3 branches joined forces to build an airplane together the airplane had to be designed such that it can serve all 3 of them. So the F-35 has to have a variant which can take off vertically. And many says that capability made the whole thing waay too complicated and the boondoggle it is. And it's not a real physical "we need otherwise we cannot fight" thing. It's a legal loophole.