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. There are limits on how much additional stock can be bought at one time through the DRIP, although these limits are usually high. Additional stock can only be bought on pre-specified dates. OCPs must be sent prior to these times, and thus you don't know at what price you are buying. Also, cash payments do not earn interest while being held by the program.