Private fields are considered to be internal to the object. So the outside world doesn't need to know about how the data is stored inside the object. This means you can easily change the internal representation of data of an object while everyone else still uses the same accessors / mutators to do its work.
It's also possible that accessors / mutators perform validation and other steps that are needed internally for the object but that you don't wish to expose to code that uses the object.
You could make all fields public but that limits your ability to change the internal structure of the object later on when a lot of code now depends on a field being named a certain way.
