From a bird's eye view we would like to give the database hints on how to package tuples into pages. That's easier if the rows can not grow arbitrarily in width

From a bird's eye view we would like to give the database hints on how to package tuples into pages. That's easier if the rows can not grow arbitrarily in width. In practice that means (and I think, MySQL did that) that arbitrary length strings or other data (Character Large Objects or Binary Large Objects) are packaged into different pages and require another seek operation, while strings/bytearrays with a maximum length can be packaged with the rest of the row. Of course, when the SQL standard was conceived, databases were much dumber than they are now and hardware was quite different. Your implementation may vary.