The spacer values are not a sequence/series. They are put on scale 1-5 to provide consistent responsiveness across the breakpoints.
Following breakpoints xs, sm, md, lg were available when these values were introduced:
// Media queries breakpoints
//
// Define the minimum and maximum dimensions at which your layout will change, adapting to different screen sizes.
// Grid system
//
// Define your custom responsive grid.
$grid-breakpoints: (
  // Extra small screen / phone
  xs: 0,
  // Small screen / phone
  sm: 34em,
  // Medium screen / tablet
  md: 48em,
  // Large screen / desktop
  lg: 62em,
  // Extra large screen / wide desktop
  xl: 75em
) !default;
If you look at the code when first time these values were introduced the classes were defined like these:
 .m-t-0  { margin-top:  0; }
 .m-t    { margin-top:  $spacer-y; }
 .m-t-md { margin-top: ($spacer-y * 1.5); }
 .m-t-lg { margin-top: ($spacer-y * 3); }
They added values .25 and .5 to the scale later on.
I think they want to provide way to scale element sizes according to size of the viewport like this:
<body class="col-7 mx-auto" style="border: 2px dashed gray;">
  <div class="fs-6 
              p-0 p-sm-3 p-md-4 p-lg-5 
              m-0 m-sm-3 m-md-4 m-lg-5 
              border border-4 border-info">
    <div class="bg-warning">Lorem ipsum dolor sit amet consectetur.</div>
  </div>
  <link href="https://cdnjs.cloudflare.com/ajax/libs/bootstrap/5.1.3/css/bootstrap.min.css" rel="stylesheet" />
</body>
 
 
Run above snippet, go in full page mode, then open Chrome Developer Tools, then click Toggle Device Toolbar` and select Dimensions: Responsive option. Resize the width. You'll notice the element scales for different viewport size i.e. when it hits the breakpoints.
Intention to add more spacing values could be:
- finer spacing options- if you want more control then you can have values which are multiples of 4px. For example, considering 1rem=16px the values can be  0, 0.25, 0.5, 1, 1.25, 1.5, 1.75, 2,...,4.
 
- vertical rhythm- For this you'll have to look at elements on your page and determine what paddings and margins you need for all of them and put the values in $spacers map.
 
Following SO questions have good discussion about spacing:
What are the rules of thumb for margins in web design? 
General rule to calculate padding