For any commercial dishwasher or glasswasher sites will ideally have a standpipe in place for the wastewater to drain to. This often means the waste outlet from the machine is below the height of the standpipe. A gravity warewasher with no pump will not be able to empty as there is insufficient pressure to force the water away. A drain pump forces the dirty wash water out of the warewasher and into the waste pipe.
A site should not require a drain pump if the drain point is below the height of the machines waste outlet.
Please see the illustration below:
A number of manufacturers fit integral drain pumps as standard on all their glasswashers and dishwashers. However, some manufacturers still give you the option. Please make sure you check your drainage before ordering so you can specify the right machine.