They are not terribly effective, but if you don't have anything better, you can use them to attack armoured vehicles. In fact I believe that's how they got their name, the Finns called them 'Molotov cocktails' because Molotov was a Soviet bigwig at the time.
I guess they tried to get them inside the hatch if it was open and in that case it would be extremely effective - a bunch of guys in a very cramped space with fuel supply nearby are in major trouble. I don't know if dropping a few Molotovs on a T34 would reliably incapacitate it or not. A modern tanks would probably be virtually immune to them otherwise insurgents would use cheap petrol bombs instead of massive IEDs.
Other than that, they are an effective terror weapon, so for example if your house is surrounded by a mob, chucking petrol bombs at them will probably slow them down. No-one wants to get burned to death.
It's hard to really immolate someone with them though, because they are slow moving, therefore easy to dodge and bottles don't tend to reliably break on people. If it lands at your feet, you will get badly scorched in the fireball and splashed with burning fluid but you are unlikely to just keel over and burn to death.
You will notice that rioters tend to use them and police don't feel the need to respond by shooting people. If rioters used guns or grenades, I think it would be a different story. So that should tell you something.