As you know there are many plugins which do somehow the same job but in different ways and with different features. I think providing a specific area to categorize similar plugins in one place would be very effective in finding those which have similar functionality and features. Searching for a plugin in wordpress.org > extend > plugin gives only limited choices.
That would gives people higher ability to chose between the right plugins from a list.
For example if I'm looking for a plugin to create forms in my website and I search the plugins directory and end up with "contact form 7", it's a good approach to show a list of other plugins nearby under we say "similar plugins" which do the same thing but with different options or features.
The same goes for widgets as well.