Not very understand about how a computer scientist will say about oracles. This is my understanding. Let's consider some other examples "without" oracles. If there are 8 boxes that have 1-8 numbers inside, no repeated numbers, and there is a box keeper who knows the number inside the box. And what we want to do is try to know which box has the number 6. Every round we will point at a box and ask the box keeper whether this box has number 6. Coming back from this example, we can see that the oracle in Grover's algorithm plays the same role as the box keeper in the example above. Oracle is not created by us, it's created by others, and the important complexity is the query complexity, i.e., how many times we need to use the oracle.
Another understanding from Nielsen and Chuang's book"
This discussion of the oracle without describing how it works in practice is rather abstract, and perhaps even puzzling. It seems as though the oracle already knows the answer to the search problem; what possible use could it be to have a quantum search algorithm based upon such oracle consultations?! The answer is that there is a distinction between knowing the solution to a search problem, and being able to recognize the solution; the crucial point is that it is possible to do the latter without necessarily being able to do the former.
That is, we can ask if the answer is right every time, for example, if we want to factorize number $n$ into two prime numbers, we can ask if number $a$ is a factor of it, this asking will spend us one resource.