People, more often that not, argue that afterburn is one of pyro’s weakest and least skillful abilities. Here I am to say the opposite. Not only can fire be relevant, but a decent tool in the pyro’s toolkit, not only to proc forced criticals.
Especially with the advent of the detonator and the scorch shot, afterburn damage has become even easier to apply than ever before. With such ease it becomes practically almost free. Splash leads to not only fire-and-forget but also igniting multiple enemies at once. With the flare gun it is more difficult to have a consistant presence with burn damage, but it also comes with the threat of high damage criticals.
People also argue that there are too many counters to afterburn. It would be better to consider afterburn a consumer of resources. Jar items used to extinguish teammates can’t be used against your team for some time. Medkits take time to respawn, and can only help one person at any time. Pyros must waste both time and ammo to extinguish. It is also possible to just light someone up again just after they extinguish themselves, just to make resources even more of a waste.
Afterburn is also a form of control. It encourages people to retreat, even if they have the health to tank it. This is especially true if they are under fire from other teammates. Their retreat also becomes predictable, they must go to the nearest source of extinguishing. Any time they spend searching for relief from burning is potential time not spent elsewhere.
It also grabs attention. Fire forces enemies to yell and scream, as well as glow brightly. This points out enemies to your team, as well as foil potential ambushing. Ask any spy.
To conclude, afterburn damage is more than meets the eye. Its potential damage easily adds up over the course of a match. Even so, more of its effect is under the surface, more psychological. It forces people to spend far more time and effort to reverse burn damage than it does to apply it.