One trigger, two payouts: each time a DRUKHARI unit from your army disembarks from a TRANSPORT, until the end of the turn, ranged weapons equipped by models in that unit have [IGNORES COVER] and melee weapons equipped by models in that unit have [LANCE].
Both halves are exactly what Drukhari infantry lack. Ignores Cover solves the problem that splinter weapons are volume rather than quality, so the squad in ruins on the objective stops getting a save it did not earn. Lance adds +1 to Wound on a charge, which is the armour penetration Wyches and Incubi have never had.
And because the rule names DRUKHARI, not a pillar, it pays your Kabalites, your Wyches, your Wracks and your mercenaries identically. That is unique in the faction and it is why a mixed collection should look here first.
The cost is structural rather than a restriction: you must own transports, and a unit only benefits on the turn it disembarks, so this is a detachment that wants to keep moving.