In a detail layout, labels are often placed to the left of the field, right-aligned.
In lists, we would most often do as in portals, placing the field label above the field.
Now, if a list for screen or print contains a sub-summary above, grouping the list items by type or any other field, how would you put the labels to describe the columns?
As an example, I have an inventory list layout. Inventory can be computers, furniture and other types/categories. So there is a sub-summary for Category. And another for Manufacturer.
Putting the labels in the header would make the headling in the sub-summary come between labels and data.