The main reason for 3D printing compared to a milled block is that you are much less limited in design, a part milled out of a block is almost universally going to be stronger then an identical 3D printed part. The potential increased strength and reduced weight comes from an optimized design not possible to manufacture with milling. Even a multi axis mill is limited in that the tool have to be able to reach the areas you want to machine without for instance the holder or spindle interfering with the part or that you get to much vibration in thin walled parts while machining.
Here is a video illustrating some of the challenges with milling complex parts and interference.
This is 60kg titanium turned into 1.9kg part, such huge differentials between the starting material and finished part is very common in aircraft structural parts.