the biggest positive is not getting flats. ish. it still punctures but it seals and you will need to put more air in it usually anyways, but you won't have to change the tube. if you do get a big whole, you can usually fix it with a similar apparatus that fixes holes in car tires actually.
they are also lighter. SOMETIMES. it depends on the bike, most mountain bikes are lighter with tubeless setups, and all fat bikes are cause those tubes are like a pound each. but road is actually heavier usually.
Another reason most mtb or fat bike riders go tubeless is that you can run lower air pressure in the tires and not worry about getting a pinch flat. you can make your ride more supple and easygoing if you decide to do so.
the only real negatives are the tires are more expensive, they sometimes SUCK to set up, and if you do end up getting a hole that is too big to patch up, you need to put a tube in it. now you're taking it off, getting gross sealant everywhere, on your hands and clothes, and pray you don't get it on your rotor.
realistically, unless you're a shop/home mechanic you don't really have to worry about the setup and the big picture are already very rare anyways.
hope this helps