there are NO advantages - none whatsoever :-)

Nobody really knows why ultimaker insist on using it.

1.75mm filament heats faster and can therefore be fed faster for higher volume throughput.
because 1.75 has become the standard you can get thousands of types, colours, materials and brands as 1.75.
3mm tends to be alot more limited.

Basically unless you intend to use a nozzle size larger than 1.75mm - there is NO reason to ever use 3mm filament :-)