At the point mostly just looking at different possibilities.
While this isn’t a great example, since it would be better to use a “round over” bit for this, but in my test model I had a fillet(round over) on one vertical corner which will mill easily with 2.5D. But for additional testing and not having a round over bit handy I added a fillet on the left and rear top edge of the model. That’s where the 3D milling comes into play. Using an STL of my 3D model of my part I can pull that in and have it mill the round over using an end mill. Not saying that’s the best way to do it, but it does work.
Using CamBam I do a hybrid where I do the 3D Milling Operation just far enough down the part and then I have a second 2.5D MOP that finishes the profile milling the part out including having auto tabs to hold the part in place so it doesn’t rattle around in the stock and cause problems.
The other place where 3D milling is more likely to be used is to create molds for casting parts. My neighbor has been doing plastic casting for years building all sorts of interesting automatons that are quite lifelike. Doing 3D milling of a mold would allow taking a 3D model and milling a mold that can then be used with Latex or Resin casting. I’ll likely do some molds for him as I gain experience. CamBam has some nice features for producing molds from STL files.
But for 99% of my own projects I probably wouldn’t be using 3D milling. It was just a test to see how the capabilities of CamBam and the other packages I was looking at compared.