The only repeated information in a related table should be the key match field, there is no other reason for any other duplicated data. There are two main reason to have a child table.
1 ) So you can have any number of related items such as notes. If you had one field in the parent table then you would only have 1 note fields, but if you related in a child table then you could have 1 or more notes.
2) If you have a very large database then it is easier to keep track if you had a different table for each pertinent task.
It is possible to work with database tables in FileMaker that contain several million records. There can be extra precautions that need to be taken, but tables of such size are manageable.