You explicitly control which students have access to your course.
You can assign usernames for your students.
You can ensure that your students' information is accurate.
Students do not have an opportunity to mistakenly create multiple usernames.
If your students already have Cengage accounts, uploading roster data will create additional accounts for students whose existing Cengage accounts don't exactly match the data you provide.
Depending on the size of your class and how you receive roster information from your school, preparing a roster file for use might be a considerable effort.
You must upload a new roster to add late additions to your class before those students can begin work in WebAssign.