They maximize revenues when p=mc, because when mc=mr they are making their greatest profit. They decide on a level of output for when mc=mr.
P=MR in this market type because the price is set in the market and firms are price takers. Given that its revenue is proportional to the output, if they increased their price they would loose money because the demand would be lower and if they reduced their price they would cheat themselves out of money they could make.
I think of the market for eggs when looking back on this chapter. While maybe every egg is a little different from the next, a egg is an egg. Those in this market selling eggs, have very little control on the price they can sell their eggs at, as the market decides the price. With so many companies in the market of selling eggs, if one raises their price, the consumer will buy another brand and still pay the market price.
Connor's Blog 