The default width of the Summary field is limiting. There is usually, in my experience, room for more info in this space as the Primary Header doesn’t normally take up the entire row even on the smallest of screen sizes. For example, if I try to use totals as the summary, say for an order record (reasonable summary data, right?), if the values get in the tens of thousands, they get truncated because the default width is not sufficient.
Having wider Summary fields, suddenly opens it up to a wide variety of usages. By allowing user specified widths, control is in the developers hands as which field displays its entire value - Primary Header or Summary.