A limited supply of money curtails inflation and value debasement similar to how USD worked when we were on a gold standard.
Discussion
The question is "how is the supply limited" specifically
Itās written into the original Bitcoin blockchain and cannot be changed without a hard fork which is highly unlikely given the amount of stakeholders that exist today. This is a good explanation from Coin Telegraph. https://cointelegraph.com/explained/can-bitcoins-hard-cap-of-21-million-be-changed