Simply put, smaller denominations make bitcoin transactions easier to carry out and allow for the reading of extremely tiny transactions.
As per the overall structure of bitcoin:
1 bitcoin (BTC) = 1,000 millibitcoins (mBTC)
1 bitcoin (BTC) = 1,000,000 microbitcoins (BTC)
1 bitcoin (BTC) = 100,000,000 satoshis