I've mostly seen this done for things where there is no prefect commercial tool because it's a small market.

For example a flight school that I work with has their own simple rental administration program. It's a small webapp with 3 screens. They use it next to a SaaS booking/planning tool, but that tool never replaced their administrative approach. Mainly because it wouldn't support local tax rules and some discount system that was in place there. So before the webapp they used paper receipts and an spreadsheet.

I think the challenge in the future with lots of these tools is going to be how they're maintained and how "ops" is done.