I have worked on scan tools. The main use of scan tools comes into picture, when you are required to model a part, which does not have definite shape, no previous drawings and so on. This comes under reverse engineering.
For modeling such a part, The first thing we need to do is scan the part by using either a mechanical scanner or laser scanner. you get a *.ibl file from the scanner. These *.ibl files should be imported to pro scan tools and use this file for creating the new geometry.
This is all about scantools. I have not used copycad. So, I can not compare this with scantools.
Ofcourse there are advantages as well as disadvantages..
The advantages are
* You can build a parametric model of the irregular shaped part.
Disadvantages are
* It takes huge amout of time even for a small part
* Lot of patience required while modeling