The higher a unit's damage output, the longer it remains "highly effective" after taking casualties. For example, if a unit had +1 on the Damage Table, above and beyond it's normal benefits for an AP 1 weapon, the odds of getting an explosion increase significantly with multiple penetrations.
With two penetrations using a Meltagun, for example, the odds of causing an "Explodes" result is normally 55%. Three penetrations would have a 70% chance to cause an explosion.
If you get a +1 bonus to the damage table, the odds of causing an Explodes result with 2 Pens goes to 75%. With Three pens, that goes to 88%.
So a Melta Squad that has 6 guys would get 4 hits on average, of which 2 would usually penetrate, if you're targeting a Land Raider. In that case, you're going from a "coin-toss" to "reliable" destruction, thanks to a bonus on the damage table. That's tactically significant, as you could be reasonably certain that a single unit of Melta-users could take out a tough target without help. Instead of committing two units to killing a LR, you should only need one, most of the time.
I mean, D-Scythes are still going to rock the socks off of anything, but Eldar Melta-Users are still quite potent in the role of Vehicle Wreckers, and quite a bit cheaper.
The significant effect is that they're much more reliable in their role, than if they didn't have a bonus.