Why cut doors and windows before skin?

[John6012]

I am in the process/planning a Teardrop and wonder why some cut the doors and windows out with the sides lying on the frame. In my opinion (and I could be wrong)< it would be just as easy for me to cut the openings after I have the sides and roof mounted, gaining access to the inside from the galley/hatch. Can I cut the openings such as this or should I cut them before skin/and mounting?

[madjack]

...it is WHOLE lot easier working in the horizontal(workbench) than the vertical. Since most will use the cutout for the door as the door(at least I did) a clean accurate cut while the work is in the vertical would be a much tougher proposition
madjack

[McTeardrops]

Cutting out the opening makes the sides and skins a bit lighter, and MUCH easier to handle. The door opening is the best place to put clamps while trying to hold up and align the skins