The bride to be veil is the piece that changes the photograph. A sash on its own says celebration; a veil says bride, from across a bar, at a distance, in the blurry group shot somebody takes at midnight.
This kit puts both in one purchase, which is the practical argument for it. A bachelorette veil bought from one shop and a sash bought from another rarely look like they belong to each other, and looking like a set is most of the effect.
Two pieces, one order, one thing to hand the bride at the start of the night. It packs down for a weekend away, and it carries the whole run of bachelorette party supplies moments, from the airport to dinner to the photo nobody planned. Whoever is hosting can wrap this as a single gift and be finished, which is usually the same person quietly holding the entire bachelorette party kit together.
Who buys the bride's bachelorette sash and veil? There is no fixed etiquette rule, but the maid of honor or whoever is hosting the bachelorette party most often supplies the bride's sash and veil. Sash-and-veil sets are sold together for exactly this reason, so one purchase covers the whole bridal look for the party.