@function v/s @mixin in Sass-lang. Which one to use?

Functions are useful specifically because they return values. Mixins are nothing like functions–they usually just provide valuable blocks of code.

Usually, there are cases where you might have to use both.

For example, if I wanted to create a long-shadow with SASS, I would call a function like so:

@function makelongshadow($color) {
  $val: 0px 0px $color;
  @for $i from 1 through 200 {
    $val: #{$val}, #{$i}px #{$i}px #{$color};
  }
  @return $val;
}

Which would then be called with this mixin:

@mixin longshadow($color) {
  text-shadow: makelongshadow($color);
}

Which provides us with the actual code.

That gets included in the element:

h1 {
    @include longshadow(darken($color, 5% ));
}