Bitcoin is famous for being a deflationary currency. Its total circulating supply is capped at 21M and can never be altered. Although many have discussed the reasons for instituting a maximum cap, few know about why Satoshi chose 21M as the benchmark.
Luckily, we have many documents from the early days of Bitcoin to better understand this question. For starters, the total number of satoshis in circulation (21M BTC x 100,000,000) is an IEEE floating-point number. This makes computation far easier which is why these numbers are commonly used in computer operating systems.