Holds steel up, does not tie anything together
A rebar chair and a dowel get mentioned in the same breath because they both show up wherever steel goes into a footing, but they solve two different problems. A dowel ties two separate pours together, a bar left standing out of a footing so a wall poured later has something to lock its own steel into. A chair does not connect anything to anything else. It just holds a bar or a grid of bars at the right height inside a single pour, so the steel sits in the middle of the concrete instead of on the ground under it. One is about linking two pours across time, the other is about positioning steel within one pour.
Height comes off the plan, not a guess
Chairs come in set heights, and the height that gets used is not picked at random on site. Concrete needs a minimum amount of cover, solid material, on every side of the steel to protect it and let it do its job, and that cover distance comes off the structural drawings the same way bar size and spacing do. A chair set too short leaves the bar too close to the bottom of the form. Too tall, and there is not enough concrete over the top of it. Either way the fix is picking the right chair height before the pour, not fixing it after.
What happens without one
Steel that gets laid straight into the bottom of a trench or a form and left there does not move up on its own once concrete hits it. It sits low, sometimes right on the dirt, with barely any concrete under it and the full cover on top. That is close to the opposite of what steel bridging a soft spot needs, since a bar with no concrete around one side has nothing holding it in a straight line, and the crack that steel is supposed to stop turning into a step in a wall shows up right where the coverage ran out.