Per Michael Daly:
Why would I want to bolt something old-fashioned on the outside of the vehicle
when it is so simple for the car makers to build something _inside_ the vehicle?

1) Because the makers may not be doing so on the vehicle you prefer.

2) So you can pick the horizontal locations. For instance, some rooftop boxes
have indents for the crossbars. If the crossbars aren't located at the
indents you have to drill new holes in the box and the box rides a couple of
inches higher. Or for carrying something like a surf ski (19-20 feet long)
where you want the thing as far back as possible to minimize windage.

3) To allow for a more generic interface. Instead of having to find a set of
racks whose posts mate to a given make/model, you can choose from any
set made for rain gutters.

4) To have a known quantity. When you drill through the vehicle's structure,
you have some feeling for how strong the attachments will be. With the maker
providing their own mount points you have to trust that their strength
will be adequate. Yes, this one is a stretch.... but I can't help recalling
Chevy's Astrovan having it's front seats/seatbelt mounts pull out some years
back in whatever standard crash test it was subjected to.
--
PeteCresswell