User blog comment:FairyRaven/Fairy Tail Chapter 427: Pitched Underground Battle, Review and Commentary/@comment-7418318-20150406093019

Actually I wasn't even happy with the Star Dress in this chapter.

We've seen Virgo bury such tunnels all on her own before, so what was the purpose of Lucy transforming? Did it make them able to bury faster? Or quieter? Or through rock Virgo couldn't have broken through on her own?

Your guess is as good as mine, since Mashima never bothered to tell us. But as it is right now it looks like she only enters Virgo-mode to show off that she can do that, even though it didn't help at all.