Any OCPs received before a trade is held without receiving interest. Some allow OCPs to be automatically deducted from bank accounts. Some companies offer IRA accounts for their DRIPs. Brokerage fees for shares bought or sold through the DRIP program are usually split between all DRIP accounts, and thus small. Some companies even pay these brokerage fees.
Some companies reinvest dividends at a discount, i.e., below market price.
Most allow additional shares to be purchased in small increments. Some DRIPs allow stock to be bought via automatic monthly withdrawals from your bank account. Encourages dollar-cost averaging. Accounting is more difficult because of frequent small purchases of shares. Remember that each dividend is a taxable event as usual.