We are planning to remove it soon.
It’s intent was to show you an aggregate of topics from all the categories and tags you’ve expressed interest in. It does this by showing topics from categories and tags you’re tracking or watching. In an earlier iteration of the sidebar you had to track or watch a tag or category in order to add it to your sidebar. Then we shifted gears and decided to allow you to add things to your sidebar without having to track them. As a side effect, that “tracked” aggregator became more decoupled from the design itself.
The idea of having the ability to have links directly to aggregate topic lists is something we will likely revisit in a completely different way in the future.