Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
Illaoi is a very strong laner, with even stronger teamfight potential. She fights well along a team that can provide the necessary crowd control.
Illaoi does not need any / much offensive items to deal great amounts of damage. Her sustain in the laning phase is decent, but she really shines in teamfights. The healing she receives from her Q is amazing.
Illaoi has some downsides that you will have to play around, her immobility is one of them. Furthermore she lacks CC, but both these downsides are compensated by her insane damage.
Illaoi fully scales on attack damage and only deals physical damage.