That's actually not an accordion but what we call a "bandoneon" down here. It is not exactly an accordion. If you notice it has buttons instead of keys, and is sounded by extending rather than compressing the air chamber. It's an instrument of German folklore origin brought here by the German immigrants to the River Plate region (Uruguay and Argentina, where tango is from), and pretty much obsolete in other fields. It became absorbed into tango, and is sort of a signature instrument of the style. I don't know too much about tango since i am not too attracted to the style, but it has a kind of spirit which is unique to this region of the world. It is very rhythmic, usually in 2/4 meter, often paired with dance. Piazzolla is one of the great experimenters of it, Carlos Gardel the most adored singer. The single most recognizable tango is "La Cumparsita" (which is Uruguayan, NOT Argentinian), which you have probably heard before. It is the typical representation of the style.