If we discard the idea of the holy, there is nothing left but the secular. Without a sense of the holy, a church has nothing left but the worldly. It may be earnest, or academic, or bureaucratic, or intense, or moralistic -- but it has lost touch with God. (Even theology can lose touch with God. If you don't believe me, read some theology books.) A consumer-model church, a coffeeshop church, a browbeater church, an academic church -- all are missing holiness. They may also be missing leadership with authority, or discipleship, or fellowship, or the kind of belonging that builds an attachment.
Holiness is a spark of glory, where we recognize divine life: that it is beautiful and pure, powerful and good. Holiness is what makes us understand that God is worthy to receive honor. Without holiness, the idea of God has no attraction -- what does God have that the world does not, if God is not holy? It is holiness, after all, which is is the the soul's desire: the genuine article of holiness, where our souls are like the still water, or a kindled flame, or a field containing a hidden treasure.
Some things are related to holiness: reverence, and respect, and honor. These are produced by recognizing the holy or the worthy; without that, there can only be counterfeits of reverence and honor and respect. Holiness inspires reverence; when the real thing is found then its counterfeits evaporate like a child's footprints on a sidewalk in August.
And holiness goes beyond duty: it shows that an act of kindness is a window of divine grace. Holiness is needed for morality to transcend mere obligation. "Holiness" is, after all, the part that transcends. Worldly goodness never really soars, as it denies there is anything beyond, to which we might reach. So often the idea of "holy", in art, is shown with wings.