It is something that I haven't seen much of outside of Brazil. It has its origins in modernist architecture, but it has caught on and become very popular in Brazil. In narrow-and-long floor plan buildings, the ground floor is open, with just the building's support columns holding it off the ground, as if on stilts, and a small entry foyer, just enough to have elevators and stairs to lead up to the building itself, being the only ground-level built structure. The columns, as a matter of style are usually cylindrical, to make the open space underneath the building seem airier and unobstructed.
Sometimes, however, the 'pilots' can be not under the main building, but under a transversal, smaller wing (see Vanda's link to Palacio Capanema). I believe this is the case with the building you describe, Porteño.
Pilotis (maybe 'colonnade'?) are usually good places to hold exhibits, fairs, events in general, as they are open-air (which makes a lot more sense in a mild-weathered country such as Brazil) but protected from rain and direct sunlight.
My username is 'mglenadel'… 'm' as in mnemonic, 'g' as in gnostic…