With the tray, the child can pour with less spilling and less getting wet. Even with a tray, though, there will always be spilling and getting wet. Be prepared.
The child below is using the tray to hold her creations. The child is making forms from moon sand. The trays provides a nice platform for her work. In fact, the forms are highlighted so much better than if they were just made on top of the sand in the table itself.
Two quick notes about this apparatus. If you are using it for water, caulk the seams with a bathroom caulk otherwise it will leak at the joints. Also, if you are using it with water, drill holes in the side so water will leak out into the table otherwise the children will fill it to overflowing. That is bad if your tray hangs over the side like the one pictured. I have had to adjust the size and number of holes depending on the creativity and enthusiasm of the group. If there are too many holes, I easily correct it with duct tape.