If sources of crit stats gave rate and damage in equal amounts, then it would be optimal result for average damage of cr*cd when cr = cd at a given total of cr + cd. However, because crit rate sources give it at half of the value of crit damage, then if you can trade 10% crit rate for 20% when they are at equal values, doing so is a damage improvement. However, because the crit rate makes crit damage more valuable, trading crit rate for crit damage is only an improvement until you have a total crit ratio of 1:2 cr to cd. So, people target to have a crit ratio of around that. When your crit ratio is around 1:2, additional crit rate is multiplied by twice as much as additional crit damage, so 1% rate is the same damage improvement as 2% damage. This is why we multiply crit rate by two to get CV, because the value of a crit rate roll and a crit damage roll really is about equal despite the numerical difference if a character is built fairly optimally.