When performing a recover by tag on a database with multiple large segments, you may want to create as many segments as needed when you are asked to create the new data file. Indeed, a recover by tag on large databases can take a long time. Creating the segments ensures that 4D Tools does not stop after the first segment is full and awaits the creation of the next segment.
Also, 4D leaves a reserved space at the beginning of the first segment. That space is used to store references to segments when they are added. When 4D runs out of that space in the first segment, it cannot create additional segments without performing a recover by tag. When you create segments early, the references are not stored in the reserved space. So, setting the size limit early will ensure that there is more room left at the beginning of the first segment to provision for references to additional segments. so this ensures that there is more space for future segment references if you are to add many other segments.