This is important for data being shared with host apps (e.g. via Share or Action Extensions). When the user selects some data type in a host app and it matches the data types define here, the host app will offer the extension. It is worth noticing the difference between this and data sharing via UIActivity where we had to define the document types, also using UTIs. An app does not need to have an extension for that. It is possible to share data using only UIActivity.