Surely we can just do this ourselves internally?

Yes, you can. But if you want to migrate your existing data into a new CMS, contemplate whether it is acceptable to simple copy it across. Think about the volumes of data that need to be migrated. Think about your regression testing strategy. Think about resourcing and business continuity. One in five CMS migration projects fail because of an underestimation of the effort involved in distilling and migrating the content.