What function do the majority of the world's 10 most populated cities serve?

It served as central business district. A central business district (CBD) is the commercial and business centre of a city. In larger cities, it is often synonymous with the city's "financial district". Geographically, it often coincides with the "city centre" or "downtown", but the two concepts are separate: many cities have a central business district located away from its commercial or cultural city centre or downtown. Both the CBD and the city centre or downtown may also coincide with the central activities district

In my honest opinion , i’d say franklin d roosevelt was a president that impacted our nation , i say this because fdr is voted among top three best us presidents and he was the longest serving president by 3 terms , usually presidents only supposed to have two terms increments of 4 years , equivalent to 8 years total before election time. and he established the fdic , through the 1933 banking act and he established the us , social security system , during his presidency, and established a national minimum wage , through the fair labor standards act , fdr led the allied coalition to victory in wwii and he played a major role in the creation of the united nations.