The addition “WITH HEADER LINE” has technically been unnecessary going back several SAP versions now. This is because the statement declares both internal tables and an additional data object – the header line.
There are a large number of notes that spread awareness that the use of this statement causes various content problems. Among other things, the use of the same name means that it is not immediately apparent as to whether you are working on a table or a header line.
However, what the notes typically do not warn you about is that this kind of programming goes hand in hand with security problems for your SAP systems.