Thanks to its tremendous typing and ridiculous bulk, Mega Meganium can put a stop to threats like Garchomp, Dragonite, Breloom, Mega Crawdaunt, Mega Cacturne, and Keldeo along with quite a few others. It also has great utility with its ability, Magic Bounce. In a metagame influenced greatly by hazards (such as Spikes, Stealth Rock, Sticky Web, etc.) keeping them off of the field is one of the most important tasks to a competitive battler, and Magic Bounce helps keep a player's side clean. This ability will also put a stop to annoying Pokémon like Sableye from carrying out their shenanigans, as Magic Bounce also bounces back status attacks and Taunt. Unlike other Magic Bounce users (such as Espeon & Xatu) it has the perfect typing and bulk to make use of this ability. Unfortunately it has a subpar support movepool with only a few viable options (Aromatherapy, Leech Seed, and Dragon Tail). Not only that, Poison moves makes quick work of it, due to the massive quadruple weakness.
Overall this is a very good Pokémon that can support teams tremendously with its bulky and ability to deny hazards.