It's just a mechanism to incentivize mining. The alternative is that miners are paid only via fees, but that risks making it prohibitively expensive to transact. Minting new coins distributes the cost of mining over all holders by inflating the currency a little bit. Fees are still necessary to avoid spamming.

Bitcoin was intended to be funded entirely by transaction fees and the minting was just to jumpstart the currency, which is why the new coin minting rate tends to zero. This was a naïve mistake. Other cryptocurrencies have start high and approach a lower nonzero value.