Bootstrap negative margin on rows

It's because the containers are meant to be used to contain any content, not just the grid rows and columns. Without padding on the container, content is forced up against the edge of the layout and doesn't align with the other content...

<div class="container px-0">
  <p>This content is aligned with the outer left edge and doesn't align with grid content.</p>
  <div class="row m-0">
      <div class="col-sm-4">
          grid content
      </div>
      <div class="col-sm-4">
          grid content
      </div>
      <div class="col-sm-4">
         grid content
      </div>
  </div>
</div>

https://codeply.com/go/23PqWB19ol

You can see several examples of container used for other than grid content the Bootstrap examples

Negative margins also work better for Responsive Design. Many people ask "why not just adjust the padding on the first and last columns?". This demo shows why

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Related: Do you need to use Bootstrap's "container" and "row" if your content is to span the whole width?

Here is your simple and easy answer

Go to your class where you want to give a negative margin and use this method.

Example for margin top

mt-n3

Example for margin bottom

mb-n2