The difference is whether the government borrows money, or simply creates it. (And yes, I am aware that borrowing money can create money, via the magic of fractional reserve banking.) When the government borrows the money, they have to pay it back, or at least pay interest on it. This puts some constraint on the government. Whereas if the government creates the money, there is no constraint - it can spend as much money as it wants. If you don't see why that might be a problem, let me ask you: Have you looked at Congress lately?
But if the government is using the money to create things of value to the society, what's the problem? That's a reasonable position - if the government is doing so. And for some government spending, that's true. But for a fair amount of it, no, it is not invested in creating things of value to society, and so no, society does not get a return from it.
Take the military, for example. It keeps other nations from destroying things of value to our society, but it doesn't create any value. (It creates jobs, and profits, but not anything of productive value.) And that means that it's a drain on society - the people who are in the service, or creating weapons, could be doing something else of more value to society - teaching children, or repairing roads and bridges, or building a better air traffic control system.
So if Congress can create money, it can divert as much of the actual productive ability of the economy - people and materials - to whatever Congress thinks will get more votes. Again, I say, look at Congress. Do you really want to give them the ability to steer unlimited amounts of the economy to do whatever they think should be done? I don't. It's a horrible idea.
I was going to answer more of your questions, but this is already long enough.
