I searched up the formats that have Iron Ball as a recommended item in a Pokemon's sample sets on Smogon's Strategy Dex. Not counting Analytic and Gyro Ball, Iron Ball has use in Gens 8 (SS), 5 (BW), and Gen 4 (DP). If this answer wasn't what you were looking for, please tell me so I can hide this answer.
Gen 8: SS 
Grimmsnarl: Uses Iron Ball in VGC22 Series 13 and VGC22 to take away useful items from Pokemon and give them Iron Ball to slow them down and make them weak to Ground moves thanks to Prankster + Trick
Indeedee-F: Uses Iron Ball in 2v2 Doubles to lower its speed so its Psychic Terrain can be setup after Rillaboom, making Grassy Surge weaker
Politoed: Uses Iron Ball in 2v2 Doubles to lower its speed so its rain can stay should it have to face an opposing weather setter not holding an Iron Ball
Gen 5: BW 
Glaceon: Uses Iron Ball in VGC12 to be able to outspeed Pokemon under Trick Room who would normally outspeed Glaceon when Trick Room is up.
Ninetales: Uses Iron Ball in VGC12 to be slower than opposing weather setters that don't have Iron Ball themselves
Gen 4: DP 
Metagross: Uses Iron Ball in DP OU for a Trick + Iron Ball Strategy to lure in Rotom-A(Rotom-Heat) and Skarmory so it can make the mentioned Pokemon weak to Metagross' Earthquake
There's one I didn't mention, which was Aegislash who used Iron Ball for its Gyro Ball in Gen 7 Ubers
TL;DR Iron Ball was used in competitive to make other Pokemon hold it and be vulnerable to Ground moves due to Trick, make the hold slower to outspeed Pokemon in Trick Room one would otherwise be unable to, and make weather/terrain setters slow to become slower than opposing weather/terrain setters not holding Iron Ball to allow their weather/terrain to stay up.
Hope this helps! :)
