When exporting an .obj file, when would you use the checkbox "as morph target"?

I can't even begin to guess what this is for. Thank you!

If I had a nickle for ever time a woman told me to get lost, I could buy Manhattan.

If you are contemplating creating a morph of the object in a 3rd party modeling software, like hexagon, Wings3D, Blender, etc.  Once you finish with the modifications, in Poser select the same obj and import your newly morphed subject into it.  A new dial is created and you just added a new morph to your object or character.

Quote - when would you use the checkbox "as morph target"?

The complete title of that check box is "As Morph Target (no world transformations)". And it is in that latter part that the biggest clue to its function lays. If you don't put a tick in this box, any transforms, that is to say translations, rotations, or scaling, that has been applied to the item will be included in the morph target. When you are exporting a morph target, most often you don't want the transforms included in the morph, so you put a tick in the "as morph target" box.

Here is a simple experiment to see the effect of ticking this box. Load a Poser box prop. create a magnet on the box and use it to make the box taller. Then xTran the box by 0.1 PNU, then export it as as an obj with a tick in the "As Morph Target" box. Then export another obj, this time without a tick in the box. Load both of the obj files you exported into the box as morphs. Try both the morphs and see the difference for yourself.

Thank you for that. I am learning a lot lately. I hope I don't overwhelm my little mind!

If I had a nickle for ever time a woman told me to get lost, I could buy Manhattan.