Yes, box springs are usable off of the floor. Unlike mattresses, box springs are hollow—minus the steel springs in certain box spring models—so they’re less likely to trap allergens or dirt the same way mattresses can.
Leaving a box spring on the floor leaves enough space between the mattress and the floor, ensuring your mattress stays clean. However, box springs aren’t the most attractive bed bases, so you might not enjoy having your box spring exposed when leaving it on the ground.