The tool you keep wishing existed.
Most businesses have one process held together by a spreadsheet that only one person fully understands, and one SaaS subscription that does about sixty percent of what was needed. Both are candidates for something built to fit.
When a spreadsheet has become the system.
A spreadsheet is the right answer for longer than people expect, and then one day it is not. The signs are consistent: only one person can safely edit it, it has rules that live in someone's head rather than in the file, and a mistake in it is expensive and hard to spot.
That is the point where the logic deserves to be software. Not a bigger spreadsheet, and not a SaaS product that assumes a business shaped differently from yours.
When you are renting something that almost fits.
Plenty of businesses pay monthly, forever, for a tool where they use one screen out of nine, and work around the rest. The subscription is rarely the real cost. The workarounds are.
If the part you actually use is well understood, it can usually be built once, shaped to how you work, and connected to the rest of your systems instead of standing apart from them. You stop renting it, and you own what replaces it.
Connected, not another island.
An app that does not talk to anything is a new silo, which is the problem we spend most of our time undoing. Anything we build reads from and writes to the systems you already have, so the app is a view onto the business rather than a separate copy of it.
That applies to staff tools and customer-facing apps alike. A field technician marking a job complete should update the schedule, the invoice, and the customer's record, because that is what actually happened.