For apparently anyone with bad vision that uses them? Lens prescription doesn't matter when glasses are materialized from magic, despite Eil initially thinking it might. Also, he can use them both as sunglasses, glasses that make night time appear day time, and thermal vision. Also, mentioned in chapter 3, but never mentioned again, his glasses work as mana batteries for himself. Also, as of the stalled translations (ch 10), numbers appear above people's heads, soon, it's explained that these numbers represent his chance of victory against that person/animal/magic beast. Also, he can parasite vision any other pairs of glasses that he's created, he only used this ability once unconsciously, and never sought to use it again. (Edit: Turns out, he used it again in 254.)

But why "Conquer the World, Probably"? Everyone else in the series is limited to the one, or rarely, multiple Talents they have. Training a second Talent is tough, because most people don't have one, they're not revealed by the testing, making them difficult to know/train. Eil's glasses let him see other people's Talents, with some conditions (I think they need to use it in front of him.) At which point, they're logged by his ability, and he can use a diminished version of them. He can only set 3 Talents at a time, but that's still hugely useful. Also, by placing glasses on a person/magic beast, he can forcefully reveal their Talent, giving himself access to that ability. He's also limited that a lot of abilities aren't nearly as useful in their initial raw state, without training in their usage or evolution through usage.