The most obvious time is when partner has made a lead-directing double, showing strength, perhaps a tenace, in dummy's suit.
But even when partner hasn't doubled, there may be other reasons. For instance, if dummy has only a few high card points, forcing dummy to take its winners early may cause declarer "transportation" problems. If your hand lends itself well be being squeezed, hammering at dummy's high cards early may be a form of anti-squeeze defense.
If you have something obvious to do, like cash a long suit or lead partner's bid suit, you'd do that. But suppose you have scattered values and no bids by your side. What would make you lead dummy's suit? Are the reasons I listed in the last paragraph valid ones? And are there others that I have overlooked?