Basically there are various differences between growing on a greenhouse bench vs growing on the floor itself.
There are basically 3 things: one is drainage.
On the bench you get good drainage out of the plant material.
When you water the top, the water can run out of the bottom of the container.
On the floor it's tend to be more restricted so crops on the floor tend to stay wetter longer.
Second thing is temperature.
On a bench you can have the air actually go under the bench so it can warm the growing media up.
When it's on the floor, it tends to be colder especially in the winter months the growing media temperatures are colder.
So again you don't get that water usage it's easier to get root disease development.
Third thing that could be an issue with growing on the floor is sometimes you get imperfections in the floor itself, so you can get pockets of water that can wick water back up into the growing media which again adds to thing staying too wet.
Overall I would say that growing on a bench is better because you get better airflow, you can get the temperature warmer right next to the root system and it'll help dry out your growing media.