There are several different reasons to use a motor plate. It is mostly considered something for "race"
In my case I have a slant six k-frame and want to install a 440 there are no conversion mounts so I do not have to find a V-8 k-frame, also even if a conversion mount is available the motor plate is less expensive

It takes the stress off the side of the block in high hp applications

It makes the removal of the engine easier in some respects

it can stiffen the front of the chassis if installed certain way.

the biggest down side is it moves the water pump forward 1/4-3/8 inch and makes the pulley alignment need adjustment