why is there no left margin for bootstrap header code example

Example: bootstrap Margin and padding

Use the margin and padding spacing utilities to control how elements and 
components are spaced and sized. Bootstrap 4 includes a five-level scale for
spacing utilities, based on a 1rem value default $spacer variable. Choose
values for all viewports (e.g., .mr-3 for margin-right: 1rem), or pick
responsive variants to target specific viewports (e.g., .mr-md-3 for 
margin-right: 1rem starting at the md breakpoint).

Margin Y 0
  
Margin Y 1
  
Margin Y 2
  
Margin Y 3
  
Margin Y 4
  
Margin Y 5
  
Margin Y Auto