What must be declared and why?
If you want to carry expensive items or quite large amount of cash, it would be better for you to declare your baggage. In this case you dont need to pay the tax fee for the import of your values because you have already declared them. If you sold or left some of your declared items in foreign country, you also need to have an official document that will confirm this action.
Besides, you can show a declaration, in case if your belongings were stolen during a trip, and prove that this items were with you before coming to a new country. In addition, you need a declaration in case if someone accuses you of stealing something that you also have.
Save all checks for the purchases in foreign country if you have declared some amount of cash.