Trace furthers its potency, allowing Gardevoir to more easily take on Pokemon like Flygon and non-Sludge Bomb Toxtricity by copying their abilities.
That description, while intended for Gen 8 UU, holds truth for practically every other tier, and in-game. Trace is good, you can copy a potentially very helpful Ability and use it to your own advantage.
Synchronize is actually quite good as well, with the potential to cripple an opponent, but Smogon generally recommends Trace.
Telepathy can be used in Double Battles, as it prevents Gardevoir from being hit by an ally's Earthquake or Surf. Telepathy seems to be Gardevoir's worst Ability.
Hope I helped!