Mega Kangaskhan’s strength derives from the mother’s happiness about her child’s growth. Watching it grow up keeps her spirits high.
– Pokémon Sun Pokédex
When Kangaskhan mega evolves, the child in its pouch grows and comes out to help it fight. Even though the child Kangaskhan is more independent, it’s still only part of Kangaskhan and not a separate Pokemon. Some of its Pokedex entries are pretty amusing, like the one from Moon: “Thanks to Mega Evolution, its child grows. But as the child is good only at fighting and nothing else, its mother feels uneasy about its future.” Or the one from UltraMoon: “When the mother sees the back of her Mega-Evolved child, it makes her think of the day when her child will inevitably leave her.”
Its ability, Parental Bond, is unique to Mega Kangaskhan. It makes single target damaging moves hit twice, with the second hit (from the child) hitting for 50% damage (reduced to 25% in Gen 7). It’s what makes Mega Kangaskhan really good, especially since it has access to Fake Out, so it can hit twice with Fake Out (which is boosted by STAB as well), doing quite a lot of damage right off the bat. Another good use of Parental Bond is with stat-boosting moves like Power-up Punch; both of its hits will increase its Attack by 1 stage each, giving it a 2-stage attack boost in one.
Stat-wise, it gains a bit in every stat when it mega evolves, with a 30 base increase in Attack, make it pretty potent with Parental Bond.