Unit testing example. More useful than assert(2,1+1 )?

Question

Software testing

the same primitive examples are everywhere
I can’t see any benefit
I don’t want to do tests for the sake of tests
I recently wrote a helper similar to

<th>{{ knp_pagination_sortable(pagination, 'Id') }}</th>

at first there were a couple of lines of code in it, then as it was used in the project, I saw cases when it was necessary to add more code to make it work,
i.e. as the project grew, cases that were not taken into account grew and I added them
before that - I just didn’t see these very ones cases
now to cover with tests for the sake of tests, I can’t force myself to
call this helper with various data, in fact assert(2,1+1 ) then assert(3,2+1) then assert(4,2+2)
for all these input data and under various conditions that I artificially come up with, it will be 99.9% green. my practice shows that the case when an error happens will most likely be detected by the customer with some of his non-trivial behavior
I see the point of doing this only when a couple of new cases are found to be implemented - then before that write tests so that they check all the previous logic in case the implementation of new cases breaks the old ones
for one I have one more question
besides the classic phpunit I am also trying now behat And that's what I really liked about it

When I eat <eat> cucumbers
  Then I should have <left> cucumbers

  Examples:
    | start | eat | left |
    |  12   |  5  |  7   |
    |  20   |  5  |  15  |

I write the function /* When I eat ARG cucumbers once
and then I can use it in hundreds of tests
how is it possible with phpunit ?
The second huge plus I noticed in behat (although it does not apply to unit testing) is that you can simply write "@javascript" and the test will be run through selenium. those. the second time you don't need to write the code, which saves a lot of time.
what problems can be with behat?
we are going to continue integration and there are rumors that it is harder with behat