Purple Pikmin special abilities:

-Slower than average

-Heavier when thrown, can't reach as far nor as high as other types 

-16 hp

-1.5 damage multiplier

-Knockback immunity

-When thrown in default mode, they create a shockwave upon impact, dealing 4 damage in a 3 block radius, and stunning entities with a 40% chance for 4 seconds 

-Carry mob throw mode: Throwing a purple Pikmin to an entity while sneaking will make the Pikmin carry said entity and follow the player at a safe distance, until the Pikmin is called by a whistle, making it drop the carried entity, or when the carried entity dies, which will make it return immediately.

Purple Pikmin have no significant immunities and are heavyweighted, which makes them both slow and hard to throw, and when these are significant drawbacks, the extra weight is hardly useless, since is not fat but hardly condensed muscle. To start the extra weight and density of purple Pikmin gives them knockback immunity and the ability to create shockwaves when thrown, which deal decent area damage and have a chance to stun, both of which easily stack and shred hordes of enemies by just spam throwing purples.
On the other hand, their buff composition gives purple Pikmin both high damage and hp, being closer to the likes of a zombie stats-wise while keeping the portability and survival advantage of a Pikmin, adding that their shockwave makes them the best fighters only matched by the white Pikmin and their deadly poison.
And lastly, this extra strength allows purple Pikmin to carry most mobs, just throwing one at an entity while sneaking will make it pick it up and start following you, easing the transport of things that normally would require boats or minecarts.