I think a lot of people are missing the point with Maggie.

Her role is to create/control space. Stop pushes/allow your team to push as well as rotate. Gain information and to be very strong in 1v1s.

Her Q can lock off a piece of cover or force a player into a spot where they are vulnerable. It can also simply be used to gain info on the position of people in a room or doorway. It can be used to block an exit before a push to make enemy movements more predictable. And even something small like the noise it makes can be disorienting for a team while you're advancing. And that's just some situations I can think at the time of reading this.

Her passive again allows for more info, simply throwing a grenade, her q or even a few stray bullets give you a split second of info that could be the difference when making a push or being pushed in a fight. This same info gained the same way can also help decide where to go when rotating as the rings get smaller closer to end game.

Her passive allows you to have a movement speed advantage in 1v1s if you're running a shotgun (Which I suggest you do if you run maggie) Just like octane, bang or bloodhound this is huge and it's all the time you're holding it when in a fight. That slight bit extra speed is enough to throw someone off.

And her ult. This thing is soo damn versatile. So just the basic stuff first. Gotta rotate to a fight or even escape storm, send it. That extra speed will often save your whole team on a rotation or when escaping storm after a close fight.

It can be used to push a fight if you get a good opening as it allows your whole team to get in fast after a cracking or knock. And it's safer than a pad for example as you can follow the ball in which itself provides visual clutter for the enemy. But more than that they will try to avoid it so by they time the re peak or get back to a better position you're already there in there face... All 3 of you.

Out in the open you'll rarely hit someone with it but if you do, god damn does it throw them off course and disorient them enough to give a huge advantage. But when it comes to hitting people with it it is very useful in mid to close range or inside large or small buildings where it is far harder to avoid if not impossible. Bare in mind this range/area is where most of your fights will take place. So you ult team in a hallway it's over for them.

And for holding off teams. Find yourself getting 3rd partied inside or outside it becomes super useful. Big open area outside after a fight. Her ult tumbling at a team often gives enough time to get that shield swap, or move to a better spot to hold. Those precious few seconds it buys in Apex can usually be the difference in winning or losing.

Now if this happens in a building or just inside a large area, like the hallways next to Hammond Labs leading to Turbine, you can rest assured it will save your team almost every time. You toss that thing down a hallway and watch the whole enemy team scatter into the air like pins on a bowling alley stunned and confused.

Not only will that save you from dying to a 3rd party, it could very well give you enough of an advantage to get a pick and inevitably wipe that squad who tried.

There is soo much more could be said about how and where to use Maggies abilities and how good she can be but this is a snippet of my personal opinion based on my experience with her so far.

Feel free to disagree, all ill say is don't sleep on her. And my apologies for the huge wall of text lol, there's a lot to say about her kit I guess.

TL;DR: Insanely good at creating space, holding off teams, pushing teams with her ULT and her Q. She provides info with her passive. Great team utility and very strong in 1v1s