This Bonus increase all the benefits the Blueprint provides even more. A benefit is when a parameter on the Blueprint is above the value of a common item, or a parameter which isn't on the corresponding common item List of Parameters.
Trying to get a high Crafting Bonus is beneficial for high Item Rarity Blueprints.
Maximal Crafting Bonus
Intelligence is used to calculate the maximal Crafting Bonus (WIP - but tested up to 620 Intelligence):
(current Intelligence - 100% Baseline Intelligence) / 20
So for a Character with 450% displayed Intelligence the maximal Crafting Bonus is 350% / 20 = 17,5%
How Crafting Bonus affects crafting
The Crafting Bonus is applied to the increases a Blueprint provides, but not the Items base stats. While crafting with more than the 100% Baseline Intelligence, the crafting bonus is picked form a range of 0 to max Crafting Bonus. After a craft the Crafting Bonus, which was applied, is displayed with one decimal place, internally an unrounded (real) value is used. All values displayed (Blueprints, Items ...) are rounded, internally real numbers are used.
For example: Crafting a weapon from a Blueprint with 150% damage, getting a 20% crafting Bonus will create a weapon with 160% damage. ((150 -100)*1,2 +100).
When mass crafting stackable Items (Building stuff like thatch wood stone, but also planks, sails etc.) the new Item will receive the Crafting Bonus of an Item which is in the receiving container. So when Medium Wood Planks are crafted in a smithy, it is beneficial to know the max Crafting Bonus, and remove crafted Planks which have a low Bonus. After getting one with almost maximal Crafting Bonus, leave this in the Smithy, all following crafts will be crafted with this Bonus.
Items crafted from Blueprints can have additional Upgrades. As the Base for the Upgrade is the value of the Parameter after the Craft, and not the one of the Blueprint, the applied Crafting Bonus is also beneficial to the Upgrades.
Here a Blueprint
And a crafted one with lowish Crafting Bonus
And another Craft, with higher Crafting Bonus and 4 Upgrades applied to Intelligence.