The main benefit to Illusion is the element of surprise. you get a free attack out of it.
so Toxicroaks Fighting/Poison typing just screams out for a Psychic attack, it also has two other weaknesses: Ground and Flying.
Zoroark being a Dark Type takes x0 Psychic, x1 Flying and x1 Ground.
now say we throw in Machamp as an Illusion.
Fighting has x2 Psychic x2 Flying.
Zoroark takes x0 Psychic and x1 Flying here.
now lets say Gengar. with Levitate Gengar takes x0 Ground, x2 Psychic and x2 Ghost.
Ziroark will take x0 Psychic, x0.5 Ghost, and x1 Ground, but due to Levitate being perceived they aren't likely to use a Ground attack.
so most likely either
A: they use a Psychic attack, you take 0 Damage and get a free attack
B: they use a Ghost attack you take 1/2 damage and get a cheap attack
OR
C: for some reason they use a Ground attack and you take neutral, likely survivable damage, and retaliate with your own attack.
