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