The problem with liquifry is that it comes as a suspension in a small bottle with a nozzle which dispenses it as drops. If you just add a drop straight to the tank, the whole drop just sinks to the bottom in a clump. If the fry are in a net, it disappears straight through the bottom of the net before the fry have chance to eat any. Mixing it with a bit of tank water first gets it to spread out more and the individual tiny particles then float gently downwards. With fry in a net, they then have plenty of time to eat it before it goes through the fabric. Just a couple of mls of water is plenty to mix it with.
It is also easier to deliver the liquifry right to the fry if they are in a tank rather than a net as you can suck the liquifry-water mixture into a pipette and squirt it right in front of the fry, which you can't do by squeezing a drop from the bottle.