What’s this framed concrete rectangle in my unfinished basement?
It is a concrete footing (foundation) for a column or vertical support. It is used to support the buildings horizontal beams. There should be one directly above it.
It differs from the basement slab (floor) in that it is constructed to support heavy loads. It has a thicker/deeper concrete base with re- enforcement bar added to it. The bolts tell you it was meant for a steel column with a plate welded to the base that the bolts went through.
Why it's not being used is hard to say. Possibly the house was re-modeled so it wasn't needed or it was a mix-up in the design plans.