Helmet ridges were popular in certain regions and eras of the Empire to deflect downward-striking blows - the popularity of the ridge waxed and waned as it was needed. When the Romans encountered the Dacians with their long, curved-blade falx, ridged helmets became very popular, as a downward strike with the falx could glance off of one’s helmet… and smoothly slide down to gouge out a poor legionary’s eye, or poke into his soft, squishy brain. The ridge there causes a blow to lose momentum, or deflect outward, rather than letting it slide inwards towards the unprotected face.