I need an explanation...

[ncba93ivyase]

What exactly is the purpose of the Full Incense, Iron Ball, and Lagging Tail? All they do is slow down your Pokémon and make them more vulnerable, but is there an advantage to training them with these items?

[Bushy]

It makes Pokemon with abilities like Levitate (Iron Ball) have a susceptibility to Ground type moves.

[ncba93ivyase]

It makes Pokemon with abilities like Levitate (Iron Ball) have a susceptibility to Ground type moves.

...But are there advantages? Like will their strength be higher when they level up?

[Necro]

Try training, it'll say something like "it gained a boosted xxxx experience" if so.

Other wise, there are some moves that do double damage requiring you taking damage before your attack, so you may wanna slow your pokemon down to do so.

[mariofreak55]

Yeah, and they help in contests.