Most plugins rely on WordPress' action hooks and filters in order to extend its features. The default archive widget uses those filters I have mentioned above (it calls 'apply_filters('filter_name',...)'). As an example, the multi-lingual plugin 'WPML' needs those to get posts only of the active language (and not all languages) by joining its own SQL table for the db query.
>> ...why exactly I need to support these things...?
You don't need to. Since you share your code I assume you'd like other people to use it. By writing it in a way to play along with other plugins, more people may use it. I find your plugin great.