Allowing a starting line in a code tag is supposed to make it easy to intermix descriptions with code, while keeping line numbers continuous across code blocks. However, this makes it hard for authors to revise their submissions (either during an edit/preview cycle, or later). If I decide to add a comment to a code example, I would need to manually adjust some number of startline attributes in subsequent code tags. Being an error-prone human, I'm liable to miss one, or get the counts wrong. Instead, I'd rather declare a code block to be a continuation of a previous code block, and leave the line counting to the underlying formatting engine.

<code>
my $widget;
...
</code>

Explanation of the first block.

<code continue>
sub foo {
    ...
}
</code>

Explanation of the second block.

knobunc describes a more elaborate version of this here.