Thanks, growstuff [smile}
As growstuff says, it's more than just a 'theory' (to call it that is really a misnomer). It's an account of how money is 'created' by governments and how it 'can' be used.
Objections citing countries like prewar Germany and Zimbabwe are pointing to countries where the power to issue money has been misused by governments. growstuff's examples of US use of its power to issue money show where it has been used for the good. It can work either way; for corruption and inflation or for stimulating an economy for the benefit of all citizens. It comes down to political choices, really.
This is what makes it so interesting for me because once we are free of the 'taxation funds spending' idea we can look at choices offered by political parties and make electoral decisions based on what we would see as a the best and fairest way of spending 'public money'. It makes a nonsense of the pre-election insistence preoccupation with 'how are you going to pay for it' which is used to denigrate party policies on spending. In fact, there seems to be only two ways that public money can be spent, corruptly or incorruptly! It offers us a clearer choice.
It also, IMO, cuts out feelings that some citizens are more 'deserving' than others because of the way they have led their lives.
If people want to find out more, Murphy's blog is a good place to start; he can see the flaws in MMT as well as the good. Or just google MMT for adherents and critiques 
e.g in this blog Murphy shows how government spending is not always particularly good for the economy
www.taxresearch.org.uk/Blog/2021/08/15/the-multiplier-shows-that-not-all-government-spending-is-equal/