In some countries, there is the risk that people add illegal drugs to your luggage in an unobserved moment.
One way to prevent that is to use lockable suitcases so that you at least have a proof when your suitcase was opened forcefully.
Normally, I travel with a backpack that has many zippers and I can't lock all of them. When traveling to India, is there any need to use a lockable suitcase instead of my backpack?