Can have in a smaller controlled environment for optimal accuracy. By not having mobility it can be of larger work envelope. Less supsceptible to damage.
Fixed CMM Con's
Parts have to be brought to it. Parts need to stabilize to the environment of the CMM before measurement.
Look at the Pro's for the fixed CMM and consider them the CON's for a portable CMM and vice versa.
In addition a portable CMM can probe a wider range of things, generally. Due to its flexibility occasionally you may run into something where you have to fixture it in order to keep it still while you probe it, but fixturing may be impossible or not an option. You would think this would be a problem with a fixed CMM as well, but if you can take a part to the CMM this shouldnt come up in the first place. The ability to be able to go anywhere and probe something with it is very useful.