A removable/adjustable visor is ideal if you ride in varied conditions or switch between disciplines. On mountain bike trails, the visor shields your eyes from sun, rain, and low-hanging branches, and you can push it up to stash goggles underneath on climbs. When you hit the road, just snap it off for a cleaner, more aerodynamic profile.
The trade-off is a small weight penalty—typically 20 to 40 grams—and a slightly more complex helmet design. Fixed visors can't adapt this way, so you're stuck with whatever angle and coverage the manufacturer chose.
If you primarily ride road or time trials, skip the visor altogether for best aerodynamics. But for mountain biking, gravel riding, or anyone who wants one helmet to do it all, a removable/adjustable visor gives you the most versatility without committing to a single setup.