What it does
Illuminate raises the encounter rate for wild Pokémon in the overworld. In battle, it does absolutely nothing. No damage modifier, no accuracy change, no Speed adjustment, no stat interaction. Zero.
This isn't 'small effect'. It's 'no effect at all'. Running Illuminate in a competitive match is functionally identical to having no ability.
Who runs it
Starmie has three possible abilities: Illuminate, Natural Cure, and (hidden) Analytic. Only the last two matter competitively.
Natural Cure heals status on switch-out — reliable and flexible. Analytic gives +30% power when moving last, which is strong in Trick Room teams. Illuminate contributes nothing.
The 14% adoption number reflects Champion sets that were documented or submitted with Illuminate on Starmie — not a competitive preference. Some players don't switch abilities when building casually, or the set was recorded as-is. That 14% tells you 'these sets exist', not 'Illuminate is a valid choice'.
How to use it
You don't. Illuminate does nothing in battle.
If your Starmie happens to be running Illuminate, just play it like a normal Starmie — fast special attacker, Water/Psychic typing, Hydro Pump or Thunderbolt or Psychic or Ice Beam depending on the set. The ability slot has zero influence on any decision you make.
If you're building a Starmie now: use Natural Cure for general play, or Analytic if your team runs Trick Room. There's no reason to keep Illuminate.
Tips & strategy
When you see Illuminate on an opponent's Starmie, ignore that line on the summary screen. It tells you nothing useful. Just treat it as a regular Starmie: base 115 Speed, special attacker, watch the item and move coverage.
If your own Starmie has Illuminate, change it when you next adjust your team. It won't cost you the current game, but swapping to a real ability gives you actual tools — Natural Cure status recovery or Analytic damage — that Illuminate never will.