When and why would you write integration/unit tests

@orest Tests are meant to test your application and give you confidence it works.

Feature tests will give you a lot of code coverage so if you’re happy with this then yes and confident it’s testing your application, there’d be little point dropping down a level to write integration and unit tests.

I work alot with statistical data and the likes and there I use unit tests to ensure my base data is correct. In the data that gets pushed to the frontend it might be rounded off (decimals) or converted to percentages, so a feature test is nice, but I don't feel confident with it.. So unit tests it is

Feature tests are usually a bit slower than a unit test so if you run a lot of slow tests it might be better to test that functionality only with one feature test and several unit tests.

Take this unit test as an example

 /**
    * @test
    */
    public function the_title_must_start_every_word_with_an_upper_case_letter()
    {
        $book = Book::factory()->make([
            'title' => 'THis iS tHe titLE'
        ]);
        $this->assertEquals('This Is The Title', $book->title);
    }

It never hits the database so it shaves of a few milliseconds every time it's ran.

 public function setTitleAttribute($value)
    {
        $this->attributes['title'] = ucwords(strtolower($value));
    }

If you are a solo developer then it might be less important to do unit tests but if you are part of a team the need to test in isolation might be greater since you all might be working with the same feature but in different classes.