Using percentiles for everything might help keep things more down to earth, regardless of whatever weirdness is actually in the game, in some people's minds. It means that PCs have a chance against the zombie, vampire, serial killer, mutant space alien, etc. without it being too powerful. Which is great for most survival horror scenarios. However, It's just as easy to keep the stats for everything no greater than 18 or 20, as well, so you're right in that there is no benefit.
One possible benefit, as in the Outbreak Undead system, is that everything is estimated from one roll. In OU, the degrees of success (every ten points below your chance to hit) tells you how much damage you do. Such as 5d6 for five levels of success. It also tells you how good you did at a task. Thus the better/lower you roll, the more success you have.
Admittedly, the benefits are more psychological than anything. I was a degree of success person for a very long time. But I finally realized that there are things easily done in BRP style D100 systems that are difficult for other systems.