Let's say for example that you've learned Thelonius Monk's "Straight No Chaser" in the key of F. But the night of the gig, the singer has a sore throat, and would rather sing in the key of Eb. I believe
@wmhill is simply pointing out that you could detune your bass to DGCF, play the song in F using your regular fingerings, and it will sound in concert Eb. Having this skill means you can accommodate the singer's request, without any mental gymnastics or changes to your technique. Or another skill that many bassists have, is to keep the same tuning, but transpose the music. Either approach is valid in my opinion/experience, and versatile bassists can do both.