What is the purpose of a collector box on a building downspout?

[CalMeacham]

@Dingbang

It’s a box installed in the downspout on the outside of a building. I Googled enough to find out that the thing is called a collector box, drain box, leader head, or collector head. What is its purpose?

The third image down on this page illustrates what I’m talking about: http://www.nosansignature.com/blog/exterior-selections/copper-gutters

I understand that they can be ornamental, as in that pic, but they also come in entirely plain designs. The one that got me wondering is on the side of a drab, three-story office building and is simply a flat box on the downspout, a few feet from the rooftop. It appears to be functional and not at all cosmetic. Why is it needed, rather than just having the water flow straight through the vertical run of the downspout?

Beats me. I don’t think I’ve ever seen one before. Certainly they’re not necessary to the proper functioning of a downspout, since every other downspout I’ve seen worked fine without one.

You don’t need it to break vacuum – rainwater never comes close to filling the cross-section of a downspout.

I vote for “Ornamental”