Shadow care can vary quite a bit--and as others have said, it depends on the mama's situation, the mw, the state, the OB....
Some families simply want to know that if they end up needing to transfer care, they have an OB already lined up and are already registered at a hospital. And/or, they want some medical stuff during pregnancy--whether 'standard labs/tests', or special things like regular titers tests if they are Rh -, other.
Some families can't find 'willing OB backup for homebirth', so they do the whole package of OB prenatal care (with an 'oops, didn't make it in time' planned).
Some find OB backup that IS willing to support homebirth. Some of those OBs only want certain labs done (minimum, with more available as parents choose it) and only a min. number of prenatal visits to have some kind of record of care and relationship established. Some of those OBs want the woman to have the full prenatal package.
Most often, IME, shadow care is to cover 'just in case of transfer'. Less often, it is for moms with special issues they want a medical eye on, during pregnancy.
Shadow care, IME, can be a real boon to a family for various reasons (if only to keep extended fam from worrying too much about hb plans)--and a real PITA, if the doc likes to use scare tactics...
So if you're thinking of getting shadow care, consider the options carefully, and make sure you know what your mw thinks so you can be as much on the same page as possible.