Just wonderin'...

It's a 'poser' method. The reason they do it on Graveyard is that it probably has some benefit when you are attempting to restore a car to factory specs with all the paint marks and clips and tags. It allows you to assemble all that goo-goo away from the car then fit it in as a package.

On anything else, it's of very dubious benefit. On the flip side, installing an engine and transmission from the top using a cherry picker (or forklift ala 99% of TV shows on Earth) is also lame.

First drop the engine in from above. Then, install the trans from under the car, on a lift if at all possible. That gives you the best in terms of ease, lowest risk of damage, and making everything fit properly.