These supply reductions occur every 210,000 blocks, or roughly every four years, as part of Bitcoin's gradual, disinflationary approach to its final capped supply in circulation.

The finite supply of 21 million coins is a, if not the, foundational characteristic of Bitcoin. This predictability of supply and inflation rate has been at the heart of what has driven demand and belief in bitcoin as a superior form of money. The regular supply halving is the mechanism by which that finite supply is ultimately enacted.

The halvings over time are the driver behind one of the most fundamental shifts of Bitcoin incentives in the long term: the move from miners being funded by newly issued coins from the coinbase subsidy — the block reward — to being funded dominantly by the transaction fee revenue from users moving bitcoin on-chain.

As Satoshi said in Section 6 (Incentives) of the whitepaper:

"The incentive can also be funded with transaction fees. If the output value of a transaction is less than its input value, the difference is a transaction fee that is added to the incentive value of the block containing the transaction. Once a predetermined number of coins have entered circulation, the incentive can transition entirely to transaction fees and be completely inflation free."

Historically the halving has correlated with a massive appreciation in the price of bitcoin, offsetting the impact of the miners' subsidy being cut in half. Miners' bills are paid in fiat, meaning that if the price of bitcoin appreciates, resulting in a larger income in dollar terms for the lower amount of bitcoin earned per block, the negative impact on mining operation is cushioned.