(I'll do my best to keep this succinct): If an item is already active in the logistics GUI, when hovering over said item in "set request" (the items highlighted red showing they already exist), the same item should be highlighted in logistics.
what
tl;dr says it all.
why
As for why, there is several reasons. Highlighting is already done when hovering over a recipe, your inventory of that item is highlighted so you can quickly see where it is. Its nice UI/UX polish that streamlines activities. This is simply applying this same concept. If your logistics screen is loaded with items, it can a bit of time trying to find an item to modify/remove, especially when the numbers can semi-obscure the icon. This is a little bit of extra polish that removes those few seconds. As shown in the image below with the red highlighted items showing they exist in logistics, hovering over that should highlight the item in logistics so you can quickly reference where it is and what its set to.
Thanks for considering my request, hopefully I was clear in communicating it. tl;dr- Its a little bit of extra polish to the UI, and will streamline finding items when its already packed with other items.