When you first place the item, it is unowned which allows any sim to add money to it. The first time a sim adds money to one of the items, they “own” it which means from that point on only they can add/take money from it. Sims with certain traits (evil, mischievous, kleptomaniac, etc.) to steal from other sim’s “piggy banks”. If the owner sim is around when money is stolen, it will affect their relationship.