75mm brakes are built specifically for narrow skis—race skis and frontside carvers with a waist width under 73mm. If you're skiing fast, tight turns on hardpack, these narrower brakes are ideal because they sit close to the ski with minimal drag, so they won't catch in the snow even during deep carves.
However, 75mm brakes will not work on wider skis. If your ski waist is 73mm or wider, the brake arms won't clear the edge of the ski when deployed—they'll sit on top of the ski instead of hanging below it. That means they can't do their job of stopping a runaway ski after a release, which is a real safety issue.
The general rule: your brake width should be your ski waist width plus 5-15mm of clearance. So 75mm brakes are the right call for skis in the roughly 60-72mm waist range. Always double-check your ski's waist width before buying—this is one spec where getting it wrong really matters.