What are the key benefits of using an expert for software implementation?

Our company is about to undertake a major software implementation project for our new enterprise resource planning (ERP) system. It's a big undertaking, and we're debating whether to rely solely on our internal IT team or to bring in external experts. I'm trying to get a clearer picture of the advantages of using specialized consultants for such a project. Has anyone been through a similar process and can speak to the key benefits they experienced from having an expert handle the implementation?