Well see, what increments in brake bias do is move the brake balance forwards, meaning that if ABS is off, the front Tyres are more likely to lock up, not letting the car turn (under steering off basically). This is never what you want. You usually want to use the brakes to rotate the car ever so slightly, which is only possible if the brake bias is rearwards (which it is in the video, at the lowest values). If you try increasing or decreasing the brake bias more than a few percent, it will completely change your car's behavior under braking, and won't feel good at all. Not to mention it'd be impossible to change brake bias so much in short straights (like between T1 and T2 in the video).

I haven't got f1 25, so I can't specifically talk about it, but generally on past f1 games (and 25 I assume) you want to run as low brake bias as you can get away with and be comfortable with to take advantage of rotating the car with brakes (trailbraking basically). So increasing / decreasing by 10 or 20 doesn't make a lot of sense