Every property buys a PMS and most buy a POS. Almost nobody buys software for the laundry, the spa diary, the roster or the expense file — so those live in notebooks and spreadsheets, and the reconciliation between them lives in somebody's head.
The argument for bringing them in is not feature count. It is that they all end up touching the same two things: the guest bill and the month's numbers. A spa treatment has to reach a folio. A roster has to reach payroll. An expense has to reach the profit and loss.
Because these modules share one billing anchor, one tax engine and one settlement path, a treatment booked at the spa desk behaves at checkout exactly like a bottle of wine from the restaurant — no second integration, no second reconciliation.
They are also individually optional. A restaurant with no rooms can take the parts that apply and never see the rest, because the gating is per feature rather than per product.