The unit tests and integration tests are for the developers. And not only for the developer working with the item being implemented, but all the developers currently in the team; and all the developers that will join the team in the future. The unit tests and integration tests are not for the customer, they are there to give the developers a safety net; there to specify the intent of the code; there to embrace quality. I would go so far to say:
a customer should never question the existance of tests, only the lack of them
Isn’t unit tests enough?
No! If you would have to select one, integrations tests is the way I would go. Why? Well I guess you don’t plan to let every piece of your code run in isolation and never integrate with other parts of your’s or other’s systems. I also guess you intend to have some sort of persistence in your system, like a database. If you don’t do integration tests and regression tests, how would you know that
implementation or change of feature X doesn’t brake feature Y
Why don’t you write tests?
One thing is for sure: “since you aren’t testdriven”. I’m not going to state that you must be testdriven to succeed, but it shure does help.
Not having a good suite of tests make me feel like an electrician working with high voltage without knowing if the power is on or off
To the customer
If you are the reason for not writing tests and you instead rely on manual testing, how much does the/these tester/s doing manual arbitrary testing cost and how much coverage do you gain from those testers?
To the developer
How does it feel to be blamed when you push some changes and a new version of the software is shipped and shortly after (hopefully) the customer or user comes with an error report:
Hey, this thing just stopped working but it worked a hour ago. How can that be? Don’t you test your stuff before releasing them?
They are for all!
The tests are mainly for the team but the effect of them provides benefits for all. For the customer and the users and the families to all involved stakeholders, since they get happy relatives coming home from job. Enough said.