There have been many hypotheses put forward to explain this phenomenon. Some believe it helps to reduce damage from turbulence wheras others believe it helps to increase movement around the leaves and helps avoid stagnation. The truth is, no one is entirely certain. However, a clue to the benefits of fenestration has come out of work done on an entirely unrelated terrestrial plant species.

The epiphytic arum commonly referred to as a Swiss cheese plant (Monstera deliciosa) also exhibits fenestrated leaves. Researchers at Indiana University in Bloomington have found that the holes in the leaves may actually help gather more light in a shaded environment. The understory of a rainforest and the underwater habitat in which the lace plant grows may be more similar in light availability than you would think. How would holes in the leaves allow the plant to gather more light?

As it turns out, a fenestrated leaf can grow much larger while still maintaining the same amount of surface area. By spreading out its surface area over a larger region, a fenestrated leaf is actually more efficient at gathering what limited light is available. More work needs to be done to see if this is truly the case for the lace plant but the idea is tantalizing to say the least. Sadly, like too much of Madagascar's wildlife, the lace plant is becoming quite rare in the wild due to habitat destruction. So, the next time you come across one of these in an aquarium store, make sure to give this plant the attention it deserves.

Further Reading: [1] [2] [3]