Every business is different, and the most productive use of a current staff will vary.  The areas in which a business needs outsourced help will also be different, as will the extent of assistance needed.  The good news is that an HR firm equipped with the right personnel and expertise is not designed to replace existing employees.  It is designed to provide them with the tools needed to do their jobs better.
For more information, take a look at our complete guide on Manging an HR team When Hiring a PEO.