The thing is that most people don't understand that the code needs to be tested anyway. If you don't test it, you are just an idiot who has no proof that the code does what it claims to do.
Now because you have to test it anyway, you can just simply spend the same time writing a unit test instead of executing the code with manually configured non-repeatable test cycles, a.k.a clicking through the UI, or sending Postman requests, etc.
Also it's kind of selfish not to make something repeatable by others.
And as someone pointed out before me, unit tests are not about testing at all. It's a documentation about the system, and what it's supposed to do. Also it's the way to stop the next person who works on the code to ruin something by not knowing the business rules.
Now because you have to test it anyway, you can just simply spend the same time writing a unit test instead of executing the code with manually configured non-repeatable test cycles, a.k.a clicking through the UI, or sending Postman requests, etc.
Also it's kind of selfish not to make something repeatable by others.
And as someone pointed out before me, unit tests are not about testing at all. It's a documentation about the system, and what it's supposed to do. Also it's the way to stop the next person who works on the code to ruin something by not knowing the business rules.