Monster bones is just a material items used for completing quest, upgrading weapon/armor or you can just sell it off to obtain extra money in case you really need money (it doesn't cost much though). However, sometimes when you pick up monster bones especially the golden ones, you will obtain egg fragment. Match all 9 color of the egg fragments (regardless of its species) to obtain either: +8 attack (9 red), +16 defense (9 blue), or +24 (9 green) as egg hatch bonus (normally you tap egg and get random bonus). While this might seems miniscule, some monster do appreciate the min/max boost regardless (for example Tigrex family are very frail thus the boost in HP is appreciated). I will not go further in detail here as this boost is for min/max er. You can just get eggs casually from dens and tap it for random egg hatch bonus and call it a day.