Cows say moo as a way to communicate with other cows and even humans.
Even though humans do not really understand a cows moo sounds the cow moos when humans are around as a way to tell them something.
The cow moos as well as a way to communicate with bulls and other cows and other animals that may be near the cow.
Another common reason for cows to moo is to find a mate when they are looking for a mate the cow will moo and will usually moo constantly.
Just like dogs bark, cats meow, humans talk in different languages etc the cow moos as a way of communicating with other cows and people.
Cows in the wild can also use their moo sounds to keep other cows safe but then again sometimes that mooing sound can attract the cows predators.