To make the categories more useful and easier to navigate. The images themselves don't provide any additional information and can already be found on the weapon pages. The vast majority of the weapon image files don't have categories (in the case of Codex screenshots) or are only in the Weapon Photos category.
The tables list all possible locations from which the weapons or there parts could have been acquired, both part and present. Deciding to remove sorties because the parts are no longer there would mean you'd also need to remove events that have concluded because they're no longer there.
Listing sorties and invasions together isn't great because you can't actually get those items from sorties anymore, but you can from invasions. I moved sorties to their own bullet point so we can delineate what is currently available and what isn't.