Taken from the website Comic Coverage by Mark Engblom:

With the exception of the Phantom (who’d been appearing in newspaper comic strips since 1936), there weren’t any “costume standards” for Jerry Siegel and Joe Shuster to follow as they created a look for Superman…so they turned to the past for inspiration. The result was a crazy-quilt of colorful wardrobe elements taken from circus acrobats, science fiction heroes, and ancient strongmen…most of which remains unchanged seventy years later. The only elements of the original costume that saw immediate change were its primitive triangular S-symbol (which continued to evolve through the 1940’s) and….the boots!
Lasting only a few issues, the original boots resembled those used by performing strongmen (who were still quite popular during the 1930’s). I use the term “boots” lightly, since they appear to be more like sandals or slippers with crisscrossed lacing to secure them to Superman’s lower leg.