Sorry, missed this comment originally somehow.  I believe this came from Garth at some point, and I think the intent was to ensure icons added by extensions remained nicely aligned after we'd made some toolbar UI change (removing the "experimental" label, perhaps?  I know text-vs-image v alignment is often finnicky).  A bunch of extensions had padding styles designed for the old layout.

Hopefully we'll move to a programmatic API for creating panels & toolbar buttons soon, which could render this code moot!