Parrying can be the key to survivability. In most systems, as your level goes up, so does your chance to hit. Eventually, hitting is almost guaranteed. In a system where a single strike can stun an opponent, this becomes a bit too powerful. Add Feats into the mix, and death is likely around any corner. Wading into battle with both sides swinging full force is a recipe for quick demise.
Parrying allows for strategic decisions. The basics is this:
Lets say we match up 2nd level Sebastiano against 5th level Arn. Both come to the table with the following stats:
Sebastiao: CR: 27, CP:3, Dex bonus d100: 15, CMR: 39
Arn: CR: 50, CP: 19, Dex bonus d100; 10, CMR: 41
Ok... the two foes come face to face and roll for initiative to see who gets first blow. I like the standard d6 for initiative roll. So I'm going to modify the CMR and give it a secondary vaule based on d100 by dividing by 5 and rounding. So now the CMRs are: Sebastiano: 39/8 Arn: 41/8
Both have a CMR of 8 on the d20 scale. The two are tied. Arn is higher level, but he is slowed down by his chain mail.
Both roll d6, and add their d20 CMR. Sebastiano rolls a 5, and Arn rolls a 4. Sebastiano wins.
Sebastiano is attacking full out. He needs a 42 (His CR + dex bonus) + 19 (Arns CP) to hit for a 61.
Arn decides he is going to parry. He parries with his CR + Dex Bonus, Subtracting 60 from Sebastiano's "to hit". Sebastiano needs a 1 or less to hit. But we're playing with the equivalent of the rule that a 1 on a d20 is an automatic hit, so he needs 01-05 on d100 to hit.
Sebastiano rolls a 12 and misses.
Arn gets two attacks per round. His second attack is at at CR of 25. He needs a 38 (25+3+dex +10) or less to hit Sebastiano. He rolls a 42, and misses.
Sebastiano now realizes he is in for it, and probably won't survive long if he keeps this up. Next round, he decides to parry Arns attack. Arn attacks full out. Arn needs a 63 to hit, and Sebastiano parries with a 42, reducing Arn's to hit do 21 or less. Arn misses with a 79, but still gets his second attatck, needing a 38 or less to hit. He rolls a 39 and barely misses.
At this point, Sebastiano should be thinking of a way out of this fight, or have some creative ideas on how to get the advantage against Arn.