As Mega Mario, Mario will be able to destroy everything in his path. He can even crush down certain Warp Pipes. In other games, it is used to crush others and slow them down.
Mario and the others can transform into a Mega form after obtaining a Mega Mushroom. One can be found in Whomp's Fortress near the sleeping Piranha Plant. After getting it, Mario is able to defeat the Piranha Plant easily and destroy Thwomps as well. On Tiny-Huge Island, Mario turns Mega again once he enters the Tiny side. Despite that they are Mega Mushrooms, they look like normal Big Mushrooms.
Mario (or Luigi) can transform into Mega Mario/Mega Luigi after obtaining the Mega Shroom found on levels. Mega Mario is helpful for destroying blocks and defeating enemies and is actually required to get certain Star Coins on levels. Mega Mario can also be used on multiplayer mode where he usually appears for the person in last place.
During the mini-game Toadstool Titan, players can transform into their Mega form after finding the Super Mushroom in a block. As a Mega, they must try to run down the other players before the transformation ends.
In Paper Mario: The Thousand-Year Door, Mario can turn into Mega Mario under the influence of a Power Punch. Also, during Bowser's games, Bowser can transform into Mega after eating two pieces of meat.
In Super Paper Mario, the Mega Mushroom returns and it can turn Mario and the others into their Mega forms. Much like in New Super Mario Bros. the Mega form can easily destroy blocks and crush any enemies in the way. This is useful in earning points quicker and leveling up.
In Mario Kart Wii, the player can transform into their Mega form after using a Mega Mushroom found in an item box during a race. When in their Mega form, the player can squish opponents and travel faster for a short period of time.
Mega Mario returns in Super Mario 3D World where his purpose is the same as in the other games.