Space, ease to use, costs saving are the main reasons. Best choice for small business or even private people who might not have a big room to dedicate to projects and prototyping, a multifunction like the FABtotum occupies less than 50cm.
It is easier to use a multipurpose device like the FABtotum as learning is required just once: as with any new technology, the first approach might be puzzling, but as fast as the user get accustomed the hardest part is done. 3D printing as well can be quite confusing, as a lot of tasks are still asked to the final user (drawing and slicing mainly): if used to CNC machines, though, everything on the FABtotum will look familiar, more than it would be using completely different set of tools.
Last but not least, an all-in-one machine allows to save budget so that it can be dedicated to projects. Add-ons will normally cost less than a whole device: this means that with a FABtotum it is possible to have a mini laboratory on a desktop.