Why 15px in .container and -15px in .row?

If you only write .row in .container, it is useless to do 15px in .container and -15px in .row.

However, .container is a container as the name suggests, so I often put something other than .row.For example, considering the example below, .container uses 15px and -15px uses .row to align the title, sentence, and in the grid.In short, it's to make it easier for developers to use.

<div class="container">
  <h2>Title </h2>
  <p> Sentences <p>
  <div class="row">
    <div class="col-sm">
      One of three columns
    </div>
    <div class="col-sm">
      One of three columns
    </div>
    <div class="col-sm">
      One of three columns
    </div>
  </div>
</div>

If you use minus margin in row, you can put plus margin elements in it.
Maybe it's because it's the purpose?

I think I can put several boxes in the row with float, but it automatically folds back cleanly
It seems to have been an ingenuity to display...

I found a site explaining that part
You can also specify a negative value!

With Bootstrap, you don't know these CSS tricks
CSS. I think it's popular because it has a beautiful layout.