Frankfurt is Germany's busiest international gateway and a strong rail hub, which makes it a practical arrival or departure point. A day exploring the old town and riverfront is worthwhile, but most travelers get more from spending the extra nights in Berlin, Munich, or a smaller city like Heidelberg.
Use Frankfurt as a connection, not a centerpiece: land there, adjust to the time zone for a day if needed, then move on to the bases that justify a longer stay.