From Brotato Wiki
Item
Dangerous Bunny is an in-game item that can be bought from a Shop or can be found in a Crate. Most Items modify the Stats of the player, and some of them provide additional effects.
Dangerous Bunny
Each Dangerous Bunny that is owned grants 1 additional free reroll in the Shop.
The free rerolls are granted for each shop, but unused free rerolls do not carry over to the next shop (i.e. they can not be saved for later).
The item count is limited to 3 per run, so at most 3 free rerolls per shop can be gained with this item.
The free rerolls always happen before the regular rerolls, and do not increase the reroll cost for subsequent (regular) rerolls.
When a Dangerous Bunny is purchased, the free reroll is granted immediately and can be used in the current shop. This will not increase the cost of later rerolls for this shop.
Also see Shop Rerolling.
The item tooltip shows how many materials were saved by the Dangerous Bunnies in the current run. This information does not take into account the material cost of the Dangerous Bunnies themselves (if they were purchased, and not found in a crate).
Strategy
Dangerous Bunny is an economic item, that effectively provides free materials over the course of the run, since it is usually beneficial to spend some materials on shop rerolls.
The initial investment (to purchase the item) is an opportunity cost that needs to be taken into account, but in most cases it is a net positive to buy this item.
Late game Dangerous Bunnies might not be able to recoup the initial investment, and should therefore not be bought (or recycled for materials, if found in a loot crate).
The item is very strong in endless mode, where it continues to provide its benefit past wave 20.