builder vs initializer

When does one want a builder of an attribute, vs an initalizer?  I'm
not sure what the difference is:

Description of 'builder' from Class::MOP::Attribute,

[...] This should be a method in the class associated with the
attribute, not a method in the attribute class itself.

Does this mean a method in My::Class vs one in Class::MOP::Attribute
or some subclass thereof?

And then in the description of 'initializer' from Class::MOP::Attribute,

[...] The initializer is used to set the attribute value on an
instance when the attribute is set during instance initialization.
[...]

Does the 'builder' _not_ set the value during instance initialization?


-- 
Ian Tegebo