Final answer:
The formation of the first Professional Basketball League in 1898 signifies the growth of professional sports and the commercialization of athletics in the United States.
Step-by-step explanation:
The formation of the first Professional Basketball League in 1898 highlights the expansion of professional sports during that time. This period saw a significant increase in the commercialization and popularity of various sports, including boxing and baseball, and marked the beginnings of professional leagues, with teams becoming focal points for community and civic identity across America. While it coincided with events like the Spanish-American War and was part of broader societal changes during the Progressive Era, the direct significance of the league's formation is more associated with the growth of professional sports as profitable entertainment.