Repeated pain is the clue
Custom software starts making sense when the same workflow wastes time every week and generic tools keep creating awkward workarounds.
Start smaller than the dream version
The first useful version should solve the highest-friction part of the workflow. That keeps the project maintainable and easier to validate.
Do not automate confusion
A messy process usually needs simplification before software. Good custom tools make decisions clearer instead of hiding confusion behind screens.