Hi Andrew!
I will try to answer some of your questions!
Superposition may be interpreted as a state of uncertainty to an observer, as you said, but this is useful indeed.
By running a quantum circuit several times and measuring the qubit state each time we can measure this probability. Suppose we have a qubit in equal superposition state (50/50%). If we run a circuit with this state 1000 times, we will get nearly 500 shots measuring 0 and nearly 500 measuring 1. Thus, we run the circuit several times to evaluate the degree of superposition.
You might think this doesn't seem very productive. Sometimes it isn't advantageous compared to using classic computers, but sometimes it is, because the way we build algorithms in quantum computing is different. Here we have some examples of problems that quantum computing provides significant speedup.
