Ballast Hopper

How does a Ballast hopper work?

It looks like from these pictures, the material comes out the sides, but I think that does not make sense.

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Sure it makes sense - as the train load of ballast travels along the routes, the ballast is dumped on either side of the tracks, to be tamped and groomed by following equipment.
Here's an example of a ballast hopper in operation on a Austrailian railway.
Here's a whole gallery with many images of ballasting operations.

I didn't have a camera handy when I saw them, but NJT has four side dump ballast hoppers parked in the Bradley Beach NJ yard along with seven gondolas. They have side dump doors, hinged at the top like a highway dump truck. They held large stone for a few days, and now they are empty.

Conrail Ortner Ballast Hopper

An Old 2 Bay covered Hopper converted into a Ballast Hopper

A BNSF Hart Ballast Hopper

CSX Orter Ballast Hopper

Another type of Conrail ballast Hopper

Finger Lakes RY Ballast hopper converted from an Old Covered Hopper