Certain arches or domes, maybe a lining for a tunnel. A tower if it’s not very windy. Really just all the stuff the Romans built, since that’s what they were working with, and their volcanic ash-based cement was somewhat weaker than modern cement.

It would be pretty hard to print between rebar. You’d need a crazy multi-axis head, and at that point it’s probably cheaper to just build a form. If they can achieve some significant strength with long fibers, which seems likely, you could spool that into the stream of concrete, but just concrete is already an actively researched problem. Printing one big form in foam or plastic and then filling it could be considered. The manual equivalent certainly makes a great building, especially in harsh climates where insulation is a concern.