They hide the internal structure of the object from the caller. This allows the internal structure to be changed without breaking a lot of unrelated code. This is called encapsulation.

For example, instead of doing

$obj->{key} = 'val';
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you'd do

$obj->key('val');
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The latter might simply do the same thing as the former. Or maybe it adds error checking. At the very least, it gives the *option* to add error checking without breaking code.