“Typically you want to iterate fast, adding/changing features as necessary. Unit test make code harder to change (you may need to fix broken tests) but also more reliable (you can be pretty sure you didn't miss anything if the unit test all pass). If you have many people working on the same code, you typically need to agree to certain things ("architecture") in advance and set it in writing as a design doc. If it is just one or two guys writing code and talking to each other, isn't that too much overhead?”