Why are there columns betweeen the upper and lower bridges?

[roadman65]

https://goo.gl/maps/PgJczSFCFcD4f8Sd6
Noticed that the upper bridge is connected to the lower bridge. Why does that need to be?

[Duke87]

The edges of the upper spans are using the spans beneath them for structural support. Notice how the lower spans have a very deep girder at the inner edge. The top span has some similarly deep girders in the middle, but a much smaller one on the edge... because the columns transfer the load down to the bridge below it instead.

I am guessing someone figured they could use a little less steel this way and thus save some money. Or at least, that would have been how it worked back when these bridges were built, since at the time the material cost of steel controlled construction cost more than labor to assemble it did, so design features that used more labor but reduced the amount of material required were common. Today, it's the other way around and newer bridges will use more material to reduce the required labor.