Yes, it is a must to get this add-on, especially in Malaysia. Since it rains regularly in Malaysia, it is safe to have your vehicles protected by insurance against floods and landslides. While it is common to assume that your car will unlikely be affected from floods and landslides, it is better to be safe than sorry. Even though you do not stay in a flood prone area, your car can be affected by flood at the malls or on highways. Water damage is not a cheap fix and will cost you a lot especially if it involves mechanical parts. Paying a small amount for the add-on is definitely worth it to get that extra protection.
In addition, this add-on also protects your car from storms. If a tree falls on your car due to storm, the insurance company will cover the damages. This is an important protection to have since there have been many incidents in Malaysia where huge trees falls over due to storm.