Yeah... True. One reason that could think of, is maybe if you have like a more complex door with many moving pieces and complex movement. In that case, youd highly reduce the amount of individual components needed in your blueprint by using a pre-combined and animated mesh. Probably its also easier to animate and handle…
Personally, I would just do these kinds of simple animations directly within a blueprint. Animating a door rig externally is just going to end up in a blueprint anyway so it's just an extra, unnecessary step, for me.