Without Negative Margin

Content up here all lines up nicely against the green border, but the social icons are "indented" from their extra padding (see the red gap next to the blue border). The extra padding or margin could also make the card look too tall vertically.

With Negative Margin

It's subtle, but all of this content lines up! Modern browsers can use the gap property with flexbox to add space between elements (just like with grid), but this technique is still helpful for compatibility.