Advantage - neutral file format can be read by any reputable CAD software.
Disadvantage - lose feature and sketch history tree in the conversion.
In Short - STL is a polygonal facets repreaentation (not accurate and not smooth) and NURB's which IGES and STEP formats represent is much more dimensionally accurate and smooth.
IGES file will turn your solid into surfaces.
STEP files will keep your solid a solid.
Parasolid files a very efficient.
STL files are also referred to as a polygonal mesh. It's like filling your object with sand and each grain is a triangle
Two popular applications, 3D printers, and Vericut verification software.
To make them smooth you can dial down the resolution to .001". Most times .01" is recommended.
But this can create a huge file, especially with non planar entities.