So are laws and human rights. Nevertheless we consider them meaningful.
What is not madey-uppy? Food. Shelter. Medicine. Services. Physical goods. Human labour.
Our system is that we use money to exchange for the above, effectively a system of deferred exchanges or IOUs.
So, while money itself may be madey-uppy, the things it buys are not and you still need money to access them. So even though it's madey-uppy you still need it.
Where madey-uppyness really matters is in deciding what money is worth. Which is where inflation comes into it.
We tend to say "this bread costs £4" as if the £4 is fixed, when really it's "this £4 is worth one loaf of bread". It's the bread that is fixed not the £.
If everyone is given an extra £ to buy bread, but no one bakes more bread because we don't actually have more wheat or bakers or fuel for the ovens, all that happens is the same bread now coat £5. Now £5 is worth one loaf of bread. A pound is still worth £1 but it is also only worth 80% what it was in terms of what it buys you.
Which is why printing money "works" to pay off debt, but also means in the non-madey-uppy world people need more of it to actually buy anything. And the lenders knowing this ask for more interest on the next loan in case the £ is worth less when they get them back. Or lend their money to someone else instead.