They are most commonly used for aggregates (sand, gravel, crushed rock). They were all over South Texas when I was down there in the 1980’s and 1990’s.
Ortners like this are used for heavy aggregates as mentioned before.
The Georgetown Rail Road (GRR) has a large fleet of these and gondolas for hauling aggregates from the quarry near Georgetown, TX down to the Port of Houston and intermediate points. This is almost the exclusive cargo for this road.
The Florida East Coast (FEC) also maintains a fleet for the same purposes.