What is Illaois job/role in a team fight? Builds?

It feels like laying the smack down on the ADC isnt excactly something she can do because of how easily she's kited. Obviously I want to focus them especially if said marksmen is actually doing damage/fed, but it sounds easier than it looks. I still think she's an awesome team fighter and such, but I can't figure out what the hell i should be doing in team fights exactly, or whether I should be engaging.

It seems like if your team decides to retreat when you'e playing her there is no turning back. I generally have to ask my team to trust me and just stick with the team fight because of how well illaoi can tank (should I even be building her tanky? I go Steraks/cleaver > tank.) It feels like actually teamfighting is a huge struggle against teams with a good mixture of range.

Ive been wanting to main her forever but I personally i feel like Illaoi's W's range is whats causing the problem of actually getting to who you want to 'Slam.' It's probably just me. She seems like she's only strong against and on certain comps. I've been told previously that the works well on counter engage comps but I barely know hat that means and I thought that Illaoi wanted to ult as many people as possible? Plus If we're waitng to counter engage wouldn't I just get poked down/ all my tentacles destroyed?

Anyways, I'm actually pretty good with her considering my division as I play her somewhat like Heim where I want to force people to fight in MY zone, and I've actually learned how to kite with her. (Sounds wierd but I've kited multiple champions from 2nd outer blue turret > red turret and turned the chase into a kill even when I've fed them.) I think she's a beautifully made champion and I love her playstyle and strengths but I really for the love of me can't figure out what her role is. Although she's one of my best champions she's the one I struggle to win with the most. I would never take her in ranked because of this even though she is one of my best top laners (mechanics wise). I'd rather not play Irelia every game even though I've probably had the most experience with her. I don't know if riot giving her tentacles slows was a good or unhealthy change when that was on the PBE but it did sound as if not getting kited to shit even by immobile marksmen like Twitch would not be as much of a problem. Help? I want her to carry me out of Bronze.